Infection with influenza A/Victoria virus in Houston families, 1976

Original antigen sin and COVID-19: implications for seasonal vaccination

INTRODUCTION

Original antigenic sin describes the phenomenon in which immunity against pathogens or antigens is shaped by the host's first exposure to a related pathogen or antigen.

AREAS COVERED

When primary immunity is boosted not by the homologous but by a cross-reacting vaccine, the newly formed antibodies may react better with the primary antigen than with the antigen actually eliciting the response. This form of immune imprinting, which has been observed with influenza, dengue, human immunodeficiency virus, and other pathogens, has profound implications for the approach to seasonal vaccination against a variety of diseases, including COVID-19.

EXPERT OPINION

Public health agencies and regulatory bodies have consistently recommended repeated vaccination every few months as a way to protect against COVID-19. However, the risks and benefits of this approach requires scrutiny given the concern for original antigenic sin in response to SARS-CoV-2. This manuscript examines what is known about immune imprinting and looks ahead to explore how this phenomenon may impact seasonal vaccination against emerging SARS-CoV-2 subvariants such as BA.4, BA.5, and BA.5.1, which have been associated increased transmissibility due to enhanced immune escape.

Household transmission of influenza A and B within a prospective cohort during the 2013-2014 and 2014-2015 seasons

People living within the same household as someone ill with influenza are at increased risk of infection. Here, we use Markov chain Monte Carlo methods to partition the hazard of influenza illness within a cohort into the hazard from the community and the hazard from the household. During the 2013‐2014 influenza season, 49 (4.7%) of the 1044 people enrolled in a community surveillance cohort had an acute respiratory illness (ARI) attributable to influenza. During the 2014‐2015 influenza season, 50 (4.7%) of the 1063 people in the cohort had an ARI attributable to influenza. The secondary attack rate from a household member was 2.3% for influenza A (H1) during 2013‐2014, 5.3% for influenza B during 2013‐2014, and 7.6% for influenza A (H3) during 2014‐2015. Living in a household with a person ill with influenza increased the risk of an ARI attributable to influenza up to 350%, depending on the season and the influenza virus circulating within the household.

Demographic buffering: titrating the effects of birth rate and imperfect immunity on epidemic dynamics

Host demography can alter the dynamics of infectious disease. In the case of perfectly immunizing infections, observations of strong sensitivity to demographic variation have been mechanistically explained through analysis of the susceptible–infected–recovered (SIR) model that assumes lifelong immunity following recovery from infection. When imperfect immunity is incorporated into this framework via the susceptible–infected–recovered–susceptible (SIRS) model, with individuals regaining full susceptibility following recovery, we show that rapid loss of immunity is predicted to buffer populations against the effects of demographic change. However, this buffering is contrary to the dependence on demography recently observed for partially immunizing infections such as rotavirus and respiratory syncytial virus. We show that this discrepancy arises from a key simplification embedded in the SIR(S) framework, namely that the potential for differential immune responses to repeat exposures is ignored. We explore the minimum additional immunological information that must be included to reflect the range of observed dependencies on demography. We show that including partial protection and lower transmission following primary infection is sufficient to capture more realistic reduced levels of buffering, in addition to changes in epidemic timing, across a range of partially and fully immunizing infections. Furthermore, our results identify key variables in this relationship, including R₀.

Estimating the life course of influenza A(H3N2) antibody responses from cross-sectional data

The immunity of a host population against specific influenza A strains can influence a number of important biological processes, from the emergence of new virus strains to the effectiveness of vaccination programmes. However, the development of an individual's long-lived antibody response to influenza A over the course of a lifetime remains poorly understood. Accurately describing this immunological process requires a fundamental understanding of how the mechanisms of boosting and cross-reactivity respond to repeated infections. Establishing the contribution of such mechanisms to antibody titres remains challenging because the aggregate effect of immune responses over a lifetime are rarely observed directly. To uncover the aggregate effect of multiple influenza infections, we developed a mechanistic model capturing both past infections and subsequent antibody responses. We estimated parameters of the model using cross-sectional antibody titres to nine different strains spanning 40 years of circulation of influenza A(H3N2) in southern China. We found that "antigenic seniority" and quickly decaying cross-reactivity were important components of the immune response, suggesting that the order in which individuals were infected with influenza strains shaped observed neutralisation titres to a particular virus. We also obtained estimates of the frequency and age distribution of influenza infection, which indicate that although infections became less frequent as individuals progressed through childhood and young adulthood, they occurred at similar rates for individuals above age 30 y. By establishing what are likely to be important mechanisms driving epochal trends in population immunity, we also identified key directions for future studies. In particular, our results highlight the need for longitudinal samples that are tested against multiple historical strains. This could lead to a better understanding of how, over the course of a lifetime, fast, transient antibody dynamics combine with the longer-term immune responses considered here.

Influenza transmission in a community during a seasonal influenza A(H3N2) outbreak (2010-2011) in Mongolia: a community-based prospective cohort study

Background

Knowledge of how influenza viruses spread in a community is important for planning and implementation of effective interventions, including social distancing measures. Households and schools are implicated as the major sites for influenza virus transmission. However, the overall picture of community transmission is not well defined during actual outbreaks. We conducted a community-based prospective cohort study to describe the transmission characteristics of influenza in Mongolia.

Methods and Findings

A total of 5,655 residents in 1,343 households were included in this cohort study. An active search for cases of influenza-like illness (ILI) was performed between October 2010 and April 2011. Data collected during a community outbreak of influenza A(H3N2) were analyzed. Total 282 ILI cases occurred during this period, and 73% of the subjects were aged <15 years. The highest attack rate (20.4%) was in those aged 1–4 years, whereas the attack rate in those aged 5–9 years was 10.8%. Fifty-one secondary cases occurred among 900 household contacts from 43 households (43 index cases), giving an overall crude household secondary attack rate (SAR) of 5.7%. SAR was significantly higher in younger household contacts (relative risk for those aged <1 year: 9.90, 1–4 years: 5.59, and 5–9 years: 6.43). We analyzed the transmission patterns among households and a community and repeated transmissions were detected between households, preschools, and schools. Children aged 1–4 years played an important role in influenza transmission in households and in the community at large. Working-age adults were also a source of influenza in households, whereas elderly cases (aged ≥65 years) had no link with household transmission.

Conclusions

Repeated transmissions between households, preschools, and schools were observed during an influenza A(H3N2) outbreak period in Mongolia, where subjects aged 1–4 years played an important role in influenza transmission.

Household transmission of Pandemic (H1N1) 2009 Virus, Taiwan

During August-November 2009, to investigate disease transmission within households in Taiwan, we recruited 87 pandemic (H1N1) 2009 patients and their household members. Overall, pandemic (H1N1) 2009 virus was transmitted to 60 (27%) of 223 household contacts. Transmission was 4× higher to children than to adults (61% vs. 15%; p<0.001).

IMMUNE LEVEL APPROACH FOR MULTIPLE STRAIN PATHOGENS

We introduce a general framework to study the dynamics of multiple strain pathogens that can impart cross–immunity through a variety of structures. We propose two types of immunity and apply them to assess the dynamics of two competing strains. We illustrate this framework using two "visions": the next–to–kin protection (NTKP) approach which assumes that strains confer cross–immunity to next in order (neighboring) strains, while providing no protection against all other strains; and the differential protection (DP) approach assumes that individuals randomly gain partial (may be reinfected) and full cross-immunity following an infection with respective probabilities. We show that the risk of infection with a particular strain is significantly higher in the DP model that the NKTP. Moreover, we demonstrate that weaker cross-immunity structures in these models are more likely to lead to instability (sustained oscillations) in the strain coexistence mode. That is, periodic oscillations are sustained in the two-strain DP model for intermediate to weak levels of cross-immunity, while the NTKP model requires at least three strains to support these unstable dynamics.

Prioritization of delayed vaccination for pandemic influenza

Limited production capacity and delays in vaccine development are major obstacles to vaccination programs that are designed to mitigate a pandemic influenza. In order to evaluate and compare the impact of various vaccination strategies during a pandemic influenza, we developed an age/risk-structured model of influenza transmission, and parameterized it with epidemiological data from the 2009 H1N1 influenza A pandemic. Our model predicts that the impact of vaccination would be considerably diminished by delays in vaccination and staggered vaccine supply. Nonetheless, prioritizing limited H1N1 vaccine to individuals with a high risk of complications, followed by school-age children, and then preschool-age children, would minimize an overall attack rate as well as hospitalizations and deaths. This vaccination scheme would maximize the benefits of vaccination by protecting the high-risk people directly, and generating indirect protection by vaccinating children who are most likely to transmit the disease.

Effect of school closure on the incidence of influenza among school-age children in Arizona

OBJECTIVE

We assessed the impact of school closures as a viable intervention in the event of an influenza pandemic.

METHODS

We evaluated the effect of scheduled, two-week winter break school closures during the 2004-2008 school years on the occurrence of influenza among children aged 5-17 years in Arizona.

RESULTS

We found a consistent pattern of benefit to school-age children during winter school closures when non-school-age children and adults experienced significant increases in influenza incidence, an increase not seen among school-age children. Quantitative analysis showed that school closures may prevent or delay as much as 42% of potential influenza cases among school-age children. In addition, the ratio of illness in school-age children as compared with adults and non-school-age children decreased significantly from before to during the same school closure periods.

CONCLUSION

This analysis provides evidence to suggest that school-age children may experience a slowing of influenza transmission during winter school closures compared with those not of school age. Federal, state, and local policy makers may consider these findings in their pandemic influenza and public health emergency preparedness planning efforts.

Household transmission of pandemic (H1N1) 2009, San Antonio, Texas, USA, April-May 2009

Transmission rates were lower than those for seasonal influenza.

To assess household transmission of pandemic (H1N1) 2009 in San Antonio, Texas, USA, during April 15–May 8, 2009, we investigated 77 households. The index case-patient was defined as the household member with the earliest onset date of symptoms of acute respiratory infection (ARI), influenza-like illness (ILI), or laboratory-confirmed pandemic (H1N1) 2009. Median interval between illness onset in index and secondary case-patients was 4 days (range 1–9 days); the index case-patient was likely to be <18 years of age (p = 0.034). The secondary attack rate was 4% for pandemic (H1N1) 2009, 9% for ILI, and 13% for ARI. The secondary attack rate was highest for children <5 years of age (8%–19%) and lowest for adults >50 years of age (4%–12%). Early in the outbreak, household transmission primarily occurred from children to other household members and was lower than the transmission rate for seasonal influenza.

Low secondary transmission of 2009 pandemic influenza A (H1N1) in households following an outbreak at a summer camp: relationship to timing of exposure

Following an outbreak of 2009 pandemic influenza A (H1N1) at a residential summer camp for boys aged 10–16 years, we assessed secondary household transmission of the novel virus after their return home. Of 212 study participants who attended camp, 49 had confirmed or probable influenza for a primary attack rate of 23%. Of 87 exposed household contacts who did not attend camp, only three instances of probable transmission were observed, for a household secondary attack rate of 3·5%. All secondary cases occurred in households where the ill camp attendee returned home 1 day after onset of illness, with an attack rate of 14·3% in household contacts in this category. Returning home after peak infectivity to others and advanced warning prior to reintegration of sick individuals into the household probably contributed to the overall low secondary attack rate observed.

Age, influenza pandemics and disease dynamics

The world is currently confronting the first influenza pandemic of the 21st century [caused by a novel pandemic influenza A (H1N1) virus]. Earlier pandemics have been characterized by age distributions that are distinct from those observed with seasonal influenza epidemics, with higher attack rates (and correspondingly increased proportionate or relative mortality) in younger individuals. While the genesis of protection against infection in older individuals during a pandemic is uncertain, differential vulnerability to infection by age has important implications for disease dynamics and control, and for choice of optimal vaccination strategies. Age-related vulnerability to infection may explain differences between school- and community-derived estimates of the reproductive number (R) for a newly emerged pandemic strain, and may also help explain the failure of a newly emerged influenza A (H1N1) virus strain to cause a pandemic in 1977. Age-related factors may also help explain variability in attack rates, and the size and impact of influenza epidemics across jurisdictions and between populations. In Canada, such effects have been observed in the apparently increased severity of outbreaks on Indigenous peoples' reserves. The implications of these patterns for vaccine allocation necessitate targeted research to understand age-related vulnerabilities early in an influenza pandemic.

The shifting demographic landscape of pandemic influenza

BACKGROUND

As Pandemic (H1N1) 2009 influenza spreads around the globe, it strikes school-age children more often than adults. Although there is some evidence of pre-existing immunity among older adults, this alone may not explain the significant gap in age-specific infection rates.

METHODS AND FINDINGS

Based on a retrospective analysis of pandemic strains of influenza from the last century, we show that school-age children typically experience the highest attack rates in primarily naive populations, with the burden shifting to adults during the subsequent season. Using a parsimonious network-based mathematical model which incorporates the changing distribution of contacts in the susceptible population, we demonstrate that new pandemic strains of influenza are expected to shift the epidemiological landscape in exactly this way.

CONCLUSIONS

Our analysis provides a simple demographic explanation for the age bias observed for H1N1/09 attack rates, and suggests that this bias may shift in coming months. These results have significant implications for the allocation of public health resources for H1N1/09 and future influenza pandemics.

Optimizing allocation for a delayed influenza vaccination campaign

During unexpected infectious disease outbreaks, public health agencies must make effective use of limited resources. Vaccine distribution may be delayed and staggered through time, as underscored by the 2009 H1N1 influenza pandemic. Using a mathematical model parametrized with data from the 2009 H1N1 pandemic, we found that optimal allocations of vaccine among people in different age groups and people with high-risk conditions depends on the schedule of vaccine availability relative to the progress of the epidemic. For the projected schedule of H1N1 vaccine availability, the optimal strategy to reduce influenza-related deaths is to initial target high-risk people, followed by school-aged children (5-17) and then young adults (18-44). The optimal strategy to minimize hospitalizations, however, is to target ages 5-44 throughout the vaccination campaign, with only a tiny amount of vaccine used on high-risk people. We find that optimizing at each vaccine release time independently does not give the overall optimal strategy. In this manuscript, we derive policy recommendations for 2009 H1N1 vaccine allocation using a mathematical model. In addition, our optimization procedures, which consider staggered releases over the entire epidemic altogether, are applicable to other outbreaks where not all supplies are available initially.

Influenza excess mortality from 1950-2000 in tropical Singapore

Introduction

Tropical regions have been shown to exhibit different influenza seasonal patterns compared to their temperate counterparts. However, there is little information about the burden of annual tropical influenza epidemics across time, and the relationship between tropical influenza epidemics compared with other regions.

Methods

Data on monthly national mortality and population was obtained from 1947 to 2003 in Singapore. To determine excess mortality for each month, we used a moving average analysis for each month from 1950 to 2000. From 1972, influenza viral surveillance data was available. Before 1972, information was obtained from serial annual government reports, peer-reviewed journal articles and press articles.

Results

The influenza pandemics of 1957 and 1968 resulted in substantial mortality. In addition, there were 20 other time points with significant excess mortality. Of the 12 periods with significant excess mortality post-1972, only one point (1988) did not correspond to a recorded influenza activity. For the 8 periods with significant excess mortality periods before 1972 excluding the pandemic years, 2 years (1951 and 1953) had newspaper reports of increased pneumonia deaths. Excess mortality could be observed in almost all periods with recorded influenza outbreaks but did not always exceed the 95% confidence limits of the baseline mortality rate.

Conclusion

Influenza epidemics were the likely cause of most excess mortality periods in post-war tropical Singapore, although not every epidemic resulted in high mortality. It is therefore important to have good influenza surveillance systems in place to detect influenza activity.

Oseltamivir for influenza postexposure prophylaxis: economic evaluation for children aged 1-12 years in the U.S

BACKGROUND

Postexposure prophylaxis (PEP) with oseltamivir (Tamiflu) has been shown to be effective and is approved in children exposed to a case of influenza in a household setting. Given limited healthcare budgets, it is important to understand the costs and cost effectiveness of PEP in children.

PURPOSE

This study aims to estimate the cost effectiveness of oseltamivir PEP for children aged 1-12 years in the U.S.

METHODS

A decision-tree model with a 1-year time horizon was used to assess the cost effectiveness of oseltamivir PEP for 10 days at approved doses compared with no prophylaxis for children aged 1-12 years who were exposed to a household index case of influenza from the U.S. societal and payer perspectives. Model inputs included U.S. influenza epidemiology data, efficacy data from oseltamivir PEP clinical trials, direct medical resource use and costs for PEP and influenza treatment derived from large U.S. databases, and indirect costs based on caregiver lost productivity. Base-case estimates were tested in extensive sensitivity analyses.

RESULTS

For the societal perspective, the model estimated 12,184 fewer cases of influenza per 100,000 children exposed and an incremental cost-effectiveness ratio of $41,452 per quality-adjusted life-year (QALY) gained. Results were most sensitive to the influenza attack rate, PEP protective efficacy, and prescribing patterns for initiating PEP. Probabilistic sensitivity analyses showed that oseltamivir PEP was likely to be cost effective for all willingness-to-pay threshold values above $34,300 per QALY gained. Results were similar for the payer perspective.

CONCLUSIONS

Although there is no official cost-effectiveness threshold in the U.S., results from the current study show that when compared with no prophylaxis, oseltamivir PEP for children has cost-effectiveness ratios similar to those of vaccines for preventing influenza.

The shifting demographic landscape of influenza

BACKGROUND

As Pandemic (H1N1) 2009 influenza spreads around the globe, it strikes school-age children more often than adults. Although there is some evidence of pre-existing immunity among older adults, this alone may not explain the significant gap in age-specific infection rates.

METHODS & FINDINGS

Based on a retrospective analysis of pandemic strains of influenza from the last century, we show that school-age children typically experience the highest attack rates in primarily naive populations, with the burden shifting to adults during the subsequent season. Using a parsimonious network-based mathematical model which incorporates the changing distribution of contacts in the susceptible population, we demonstrate that new pandemic strains of influenza are expected to shift the epidemiological landscape in exactly this way.

CONCLUSIONS

Our results provide a simple demographic explanation for the age bias observed for H1N1/09 attack rates, and a prediction that this bias will shift in coming months. These results also have significant implications for the allocation of public health resources including vaccine distribution policies.

Qualitative analysis of the level of cross-protection between epidemic waves of the 1918-1919 influenza pandemic

The 1918-1919 influenza pandemic was composed of multiple waves within a period of nine months in several regions of the world. Increasing our understanding of the mechanisms responsible for this multi-wave profile has important public health implications. We model the transmission dynamics of two strains of influenza interacting via cross-immunity to simulate two temporal waves of influenza and explore the impact of the basic reproduction number, as a measure of transmissibility associated to each influenza strain, cross-immunity and the timing of the onset of the second influenza epidemic on the pandemic profile. We use time series of case notifications during the 1918 influenza pandemic in Geneva, Switzerland, for illustration. We calibrate our mathematical model to the initial wave of infection to estimate the basic reproduction number of the first wave and the corresponding timing of onset of the second influenza variant. We use this information to explore the impact of cross-immunity levels on the dynamics of the second wave of influenza. Our results for the 1918 pandemic in Geneva, Switzerland, indicate that a second wave can occur whenever R 01 < 1.5 or when cross-immunity levels are less than 0.58 for our estimated R 02 of 2.4. We also explore qualitatively profiles of two-wave pandemics and compare them with real temporal profiles of the 1918 influenza pandemic in other regions of the world including several Scandinavian cities, New York City, England and Wales, and Sydney, Australia. Pandemic profiles are classified into three broad categories namely "right-handed", "left-handed", and "M-shape". Our results indicate that avoiding a second influenza epidemic is plausible given sufficient levels of cross-protection are attained via natural infection during an early (herald) wave of infection or vaccination campaigns prior to a second wave. Furthermore, interventions aimed at mitigating the first pandemic wave may be counterproductive by increasing the chances of a second wave of infection that could potentially be more virulent than the first.

Optimizing influenza vaccine distribution

The criteria to assess public health policies are fundamental to policy optimization. Using a model parametrized with survey-based contact data and mortality data from influenza pandemics, we determined optimal vaccine allocation for five outcome measures: deaths, infections, years of life lost, contingent valuation, and economic costs. We find that optimal vaccination is achieved by prioritization of schoolchildren and adults aged 30 to 39 years. Schoolchildren are most responsible for transmission, and their parents serve as bridges to the rest of the population. Our results indicate that consideration of age-specific transmission dynamics is paramount to the optimal allocation of influenza vaccines. We also found that previous and new recommendations from the U.S. Centers for Disease Control and Prevention both for the novel swine-origin influenza and, particularly, for seasonal influenza, are suboptimal for all outcome measures.

The cost of community-managed viral respiratory illnesses in a cohort of healthy preschool-aged children

BACKGROUND

Acute respiratory illnesses (ARIs) during childhood are often caused by respiratory viruses, result in significant morbidity, and have associated costs for families and society. Despite their ubiquity, there is a lack of interdisciplinary epidemiologic and economic research that has collected primary impact data, particularly associated with indirect costs, from families during ARIs in children.

METHODS

We conducted a 12-month cohort study in 234 preschool children with impact diary recording and PCR testing of nose-throat swabs for viruses during an ARI. We used applied values to estimate a virus-specific mean cost of ARIs.

RESULTS

Impact diaries were available for 72% (523/725) of community-managed illnesses between January 2003 and January 2004. The mean cost of ARIs was AU$309 (95% confidence interval $263 to $354). Influenza illnesses had a mean cost of $904, compared with RSV, $304, the next most expensive single-virus illness, although confidence intervals overlapped. Mean carer time away from usual activity per day was two hours for influenza ARIs and between 30 and 45 minutes for all other ARI categories.

CONCLUSION

From a societal perspective, community-managed ARIs are a significant cost burden on families and society. The point estimate of the mean cost of community-managed influenza illnesses in healthy preschool aged children is three times greater than those illnesses caused by RSV and other respiratory viruses. Indirect costs, particularly carer time away from usual activity, are the key cost drivers for ARIs in children. The use of parent-collected specimens may enhance ARI surveillance and reduce any potential Hawthorne effect caused by compliance with study procedures. These findings reinforce the need for further integrated epidemiologic and economic research of ARIs in children to allow for comprehensive cost-effectiveness assessments of preventive and therapeutic options.

Long-standing influenza vaccination policy is in accord with individual self-interest but not with the utilitarian optimum

Influenza vaccination is vital for reducing infection-mediated morbidity and mortality. To maximize effectiveness, vaccination programs must anticipate the effects of public perceptions and attitudes on voluntary adherence. A vaccine allocation strategy that is optimal for the population is not necessarily optimal for an individual. For epidemic influenza, the elderly have the greatest risk of influenza mortality, yet children are responsible for most of the transmission. The long-standing recommendations of the Centers for Disease Control follow the dictates of individual self-interest and prioritize the elderly for vaccination. However, preferentially vaccinating children may dramatically reduce community-wide influenza transmission. A potential obstacle to this is that the personal utility of vaccination is lower for children than it is for the elderly. We parameterize an epidemiological game-theoretic model of influenza vaccination with questionnaire data on actual perceptions of influenza and its vaccine to compare Nash equilibria vaccination strategies driven by self-interest with utilitarian strategies for both epidemic and pandemic influenza. Our results reveal possible strategies to bring Nash and utilitarian vaccination levels into alignment.

Model answers or trivial pursuits? The role of mathematical models in influenza pandemic preparedness planning

The panzootic of H5N1 influenza in birds has raised concerns that the virus will mutate to spread more readily in people, leading to a human pandemic. Mathematical models have been used to interpret past pandemics and outbreaks, and to thus model possible future pandemic scenarios and interventions. We review historical influenza outbreak and transmission data, and discuss the way in which modellers have used such sources to inform model structure and assumptions. We suggest that urban attack rates in the 1918-1919 pandemic were constrained by prior immunity, that R(0) for influenza is higher than often assumed, and that control of any future pandemic could be difficult in the absence of significant prior immunity. In future, modelling assumptions, parameter estimates and conclusions should be tested against as many relevant data sets as possible. To this end, we encourage researchers to access FluWeb, an on-line influenza database of historical pandemics and outbreaks.

Detection and identification of echovirus 7 from a child with gastro-enteritis

BACKGROUND

Gastro-enteritis continues to be a significant cause of mortality in infants and young children in developing countries. Some previous studies have associated echoviruses with gastroenteritis.

OBJECTIVE

To look for other viral agents causing gastro-enteritis in stool samples of infants and young children admitted with diarrhoea in an urban hospital in Kenya by electron microscopy.

DESIGN

A cross sectional study.

SETTING

Mbagathi District Hospital, Nairobi, Kenya.

SUBJECTS

One hundred infants and young children admitted in the hospital due to gastro-enteritis and found to be negative for group A rotaviruses.

RESULTS

We report a virus designated KE/CVR-99 which was recovered from the faeces of a one year old male child hospitalised for gastro-enteritis. Virus particles measuring 20-25 nm in diameter were detected by electron microscopy from the stool of the child hospitalised with gastro-enteritis. After injecting the viral particles intracerebrally in newborn suckling mice which were then observed daily for signs of illness, no sign of paralysis or any other form of illness was detected. Cytopathic effect was observed in rhabdomyosarcoma cells within six days of incubation whereas no cytopathic effect was observed in L-20 B cells. We identified the virus after typing against known anti-sera to a panel of enteroviruses. This virus was found to be Echovirus 7.

CONCLUSION

This is the first case report of echovirus 7 in association with gastro-enteritis and detection by electron microscopy in Kenya.

A comparative analysis of influenza vaccination programs

BACKGROUND

The threat of avian influenza and the 2004-2005 influenza vaccine supply shortage in the United States have sparked a debate about optimal vaccination strategies to reduce the burden of morbidity and mortality caused by the influenza virus.

METHODS AND FINDINGS

We present a comparative analysis of two classes of suggested vaccination strategies: mortality-based strategies that target high-risk populations and morbidity-based strategies that target high-prevalence populations. Applying the methods of contact network epidemiology to a model of disease transmission in a large urban population, we assume that vaccine supplies are limited and then evaluate the efficacy of these strategies across a wide range of viral transmission rates and for two different age-specific mortality distributions. We find that the optimal strategy depends critically on the viral transmission level (reproductive rate) of the virus: morbidity-based strategies outperform mortality-based strategies for moderately transmissible strains, while the reverse is true for highly transmissible strains. These results hold for a range of mortality rates reported for prior influenza epidemics and pandemics. Furthermore, we show that vaccination delays and multiple introductions of disease into the community have a more detrimental impact on morbidity-based strategies than mortality-based strategies.

CONCLUSIONS

If public health officials have reasonable estimates of the viral transmission rate and the frequency of new introductions into the community prior to an outbreak, then these methods can guide the design of optimal vaccination priorities. When such information is unreliable or not available, as is often the case, this study recommends mortality-based vaccination priorities.

On the role of cross-immunity and vaccines on the survival of less fit flu-strains

A pathogen's route to survival involves various mechanisms including its ability to invade (host's susceptibility) and its reproductive success within an invaded host ("infectiousness"). The immunological history of an individual often plays an important role in reducing host susceptibility or it helps the host mount a faster immunological response de facto reducing infectiousness. The cross-immunity generated by prior infections to influenza A strains from the same subtype provide a significant example. The results of this paper are based on the analytical study of a two-strain epidemic model that incorporates host isolation (during primary infection) and cross-immunity to study the role of invasion mediated cross-immunity in a population where a precursor related strain (within the same subtype, i.e. H3N2, H1N1) has already become established. An uncertainty and sensitivity analysis is carried out on the ability of the invading strain to survive for given cross-immunity levels. Our findings indicate that it is possible to support coexistence even in the case when invading strains are "unfit", that is, when the basic reproduction number of the invading strain is less than one. However, such scenarios are possible only in the presence of isolation. That is, appropriate increments in isolation rates and weak cross-immunity can facilitate the survival of less fit strains. The development of "flu" vaccines that minimally enhance herd cross-immunity levels may, by increasing genotype diversity, help facilitate the generation and survival of novel strains.

Predictability and preparedness in influenza control

The threat of pandemic human influenza looms as we survey the ongoing avian influenza pandemic and wonder if and when it will jump species. What are the risks and how can we plan? The nub of the problem lies in the inherent variability of the virus, which makes prediction difficult. However, it is not impossible; mathematical models can help determine and quantify critical parameters and thresholds in the relationships of those parameters, even if the relationships are nonlinear and obscure to simple reasoning. Mathematical models can derive estimates for the levels of drug stockpiles needed to buy time, how and when to modify vaccines, whom to target with vaccines and drugs, and when to enforce quarantine measures. Regardless, the models used for pandemic planning must be tested, and for this we must continue to gather data, not just for exceptional scenarios but also for seasonal influenza.

Values for preventing influenza-related morbidity and vaccine adverse events in children

Background

Influenza vaccination recently has been recommended for children 6–23 months old, but is not currently recommended for routine use in non-high-risk older children. Information on disease impact, costs, benefits, risks, and community preferences could help guide decisions about which age and risk groups should be vaccinated and strategies for improving coverage. The objective of this study was to measure preferences and willingness-to-pay for changes in health-related quality of life associated with uncomplicated influenza and two rarely-occurring vaccination-related adverse events (anaphylaxis and Guillain-Barré syndrome) in children.

Methods

We conducted telephone interviews with adult members selected at random from a large New England HMO (n = 112). Respondents were given descriptions of four health outcomes: uncomplicated influenza in a hypothetical 1-year-old child of their own, uncomplicated influenza in a hypothetical 14-year-old child of their own, anaphylaxis following vaccination, and Guillain-Barré syndrome. "Uncomplicated influenza" did not require a physician's visit or hospitalization. Preferences (values) for these health outcomes were measured using time-tradeoff and willingness-to-pay questions. Time-tradeoff questions asked the adult to assume they had a child and to consider how much time from the end of their own life they would be willing to surrender to avoid the health outcome in the child.

Results

Respondents said they would give a median of zero days of their lives to prevent an episode of uncomplicated influenza in either their (hypothetical) 1-year-old or 14-year-old, 30 days to prevent an episode of vaccination-related anaphylaxis, and 3 years to prevent a vaccination-related case of Guillain-Barré syndrome. Median willingness-to-pay to prevent uncomplicated influenza in a 1-year-old was $175, uncomplicated influenza in a 14-year-old was $100, anaphylaxis $400, and Guillain-Barré syndrome $4000. The median willingness-to-pay for an influenza vaccination for their children with no risk of anaphylaxis or Guillain-Barré syndrome was $50 and $100, respectively.

Conclusion

Most respondents said they would not be willing to trade any time from their own lives to prevent uncomplicated influenza in a child of their own, and the time traded did not vary by the age of the hypothetical affected child. However, adults did indicate a willingness-to-pay to prevent uncomplicated influenza in children, and that they would give more money to prevent the illness in a 1-year-old than in a 14-year-old. Respondents also indicated a willingness to pay a premium for a vaccine without any risk of severe complications.

Incidence of influenza and associated illness in children aged 0-19 years: a systematic review

A systematic review was conducted to estimate the incidence of influenza and concomitant morbidity and mortality in children from 0 to 19 years (0-19 years). Medline was searched for observational studies and placebo or non-treated arms of experimental studies providing occurrence rates of laboratory-proven influenza illness. From the 2758 titles identified, 356 full papers were reviewed based on the abstract or title; after searching their reference lists an additional 16 papers were found. Finally 28 studies met our inclusion criteria, reporting a varying seasonal incidence of influenza of up to 46%. However, when analysing two long-term observational studies and averaging seasonal fluctuations, the overall incidence of influenza was found to range from 5% to 9.5% per year. Serious morbidity was seldom reported and no cases of mortality were found. Our review shows influenza as an infection with a moderate average incidence and a self-limiting character that is associated with mild morbidity and rare cases of mortality in children. This systematic review may be subject to two contrasting biases. First, the limited number of children reported in the literature with proven influenza infections would tend to under-represent the incidence of uncommon but serious complications such as death. Second, the preferential reporting of influenza outbreak seasons may over-estimate the importance of influenza. Future population based studies should focus on consecutive influenza seasons and capture indirect benefits of vaccinations such as interruption of transmission in the community so that preventive strategies for children can be reconsidered with special attention to their cost effectiveness.

Burden of influenza in children: preliminary data from a pilot survey network on community diseases

BACKGROUND

The burden of influenza and its clinical presentation are both poorly documented in French children.

METHODS

To measure the prevalence of acute respiratory infections over two winters (2000 to 2001 and 2001 to 2002), we performed a prospective community survey of 0- to 15-year-olds (n = 11 500 and 40 000, respectively) through a network [Grippe et Infections Respiratoires Aiguës Pédiatriques (GIRAP)] of general practitioners and pediatricians. Influenza viruses were detected by antigen detection and culture from nasopharyngeal swabs. Data on the clinical presentation of children infected with influenza A/H1N1, A/H3N2 or B viruses were derived from standardized forms.

RESULTS

The prevalences of symptomatic acute respiratory infections were 28% and 27% for 2000 to 2001 and 2001 to 2002, respectively. In the winter of 2000 to 2001, a mild outbreak of influenza A/H1N1 affected 5.4% of the study population. The attack rates of influenza A/H1N1 were highest in children 5 to 15 years of age. In the winter of 2001 to 2002, influenza activity remained at a subepidemic level (5.9%; A/H3N2, 60%; B, 40%). The incidence of influenza A/H3N2 infections was highest in 2- to 4-year-old children, whereas influenza B strains were most frequently detected in children 4 to 10 years of age.

CONCLUSIONS

Despite subepidemic levels of influenza, the GIRAP network was able to detect influenza A or B viruses in 35 to 60% of the samples during the peak of influenza epidemic. The frequency of influenza virus excretion in children with mild clinical symptoms indicates that children are an important reservoir and source of transmission of this virus.

Outbreak of influenza type A (H1N1) in Iporanga, São Paulo State, Brazil

From June to July 1999 an outbreak of acute respiratory illness occurred in the town of Iporanga. Out of a total of 4,837 inhabitants, 324 cases were notified to the Regional Surveillance Service. Influenza virus was isolated from 57.1% of the collected samples and 100% seroconversion to influenza A (H1N1) was obtained in 20 paired sera tested. The isolates were related to the A/Bayern/07/95 strain (H1N1). The percentages of cases notified during the outbreak were 28.4%, 29.0%, 20.7%, 6.2% and 15.7% in the age groups of 0-4, 5-9, 10-14, 15-19 and older than 20 years, respectively. The highest proportion of positives was observed among children younger than 14 years and no cases were notified in people older than 65 years, none of whom had been recently vaccinated against influenza. These findings suggest a significant vaccine protection against A/Bayern/7/95, the H1 component included in the 1997-98 influenza vaccine for elderly people. This viral strain is antigenically and genetically related to A/Beijing/262/95, the H1 component of the 1999 vaccine. Vaccines containing A/Beijing/262/95 (H1N1) stimulated post-immunization hemagglutination inhibition antibodies equivalent in frequency and titre to both A/Beijing/262/95-like and A/Bayern/7/95-like viruses. Thus, this investigation demonstrates the effectiveness of vaccination against influenza virus in the elderly.

Influenza: vaccination and treatment

Few conditions exert such an enormous toll of absenteeism, suffering, medical consultations, hospitalization, death and economic loss as influenza. Patients at high risk of complications and mortality include the elderly and those with pre-existing cardiopulmonary disease. The outbreak in 1997 in Hong Kong, of avian H5N1 influenza in man, which resulted in six deaths among 18 hospitalized cases, and the recent isolation of H9N2 viruses from two children in Hong Kong, are reminders that preparation must be made for the next pandemic. Since the 1970s, efforts to control influenza have mostly focussed on the split product and surface antigen vaccines. These vaccines are of proven efficacy in healthy adults and are effective in elderly people with and without medical conditions putting them at high risk of complications and death following influenza infection. However, vaccine coverage is patchy and often low, and outbreaks of influenza are not uncommon in well-immunized residents of nursing homes. New vaccines and methods of vaccine delivery are being developed in attempts to overcome the limitations of existing vaccines. The antiviral drugs amantadine and rimantadine were developed in the 1960s, but have not been used widely due to their spectrum of activity, rapid emergence of resistance, and adverse effects associated with amantadine. The site of enzyme activity of the influenza neuraminidase is highly conserved between types, subtypes and strains of influenza and has emerged as the target of an exciting new class of antiviral agents that are effective both prophylactically and as therapy.

Global epidemiology of influenza: past and present

Pandemics are the most dramatic presentation of influenza. Three have occurred in the twentieth century: the 1918 H1N1 pandemic, the 1957 H2N2 pandemic, and the 1968 H3N2 pandemic. The tools of molecular epidemiology have been applied in an attempt to determine the origin of pandemic viruses and to understand what made them such successful pathogens. An excellent example of this avenue of research is the recent phylogenetic analysis of genes of the virus that caused the devastating 1918 pandemic. This analysis has been used to identify evolutionarily related influenza virus genes as a clue to the source of the pandemic of 1918. Molecular methods have been used to investigate the avian H5N1 and H9N2 influenza viruses that recently infected humans in Hong Kong. Antigenic, genetic, and epidemiologic analyses have also furthered our understanding of interpandemic influenza. Although many questions remain, advances of the past two decades have demonstrated that several widely held concepts concerning the global epidemiology of influenza were false.

Zanamivir for treatment of symptomatic influenza A and B infection in children five to twelve years of age: a randomized controlled trial

BACKGROUND

Influenza infection rates are higher in children than in other age groups. This study evaluated the efficacy, safety and tolerability of a 5-day course of twice daily inhaled zanamivir, 10 mg, compared with placebo in the treatment of symptomatic influenza A and B viral infections among children 5 to 12 years of age.

METHODS

This double blind, randomized, placebo-controlled, parallel group, multicenter study conducted in the Northern Hemisphere during the 1998 and 1999 influenza season enrolled 471 patients with influenza-like symptoms for < or = 36 h. Patients were randomly assigned to zanamivir (n = 224) or placebo (n = 247). Symptoms were recorded on diary cards twice daily during treatment, for 9 days after treatment and for 14 additional days (if still reporting moderate/severe cough and/or taking relief medication).

FINDINGS

A total of 346 (73%) patients were influenza-positive by culture, serology or polymerase chain reaction (65% influenza A, 35% influenza B). Zanamivir reduced the median time to symptom alleviation by 1.25 days compared with placebo among patients with confirmed influenza infection (P < 0.001). Zanamivir-treated patients returned to normal activities significantly faster and took significantly fewer relief medications than placebo-treated patients. Zanamivir was well-tolerated, demonstrating adverse event profiles similar to those of placebo and no clinically significant changes in laboratory findings. Viral susceptibility testing revealed no zanamivir-resistant strains of influenza A or B.

CONCLUSIONS

Zanamivir was effective in shortening the duration and severity of influenza symptoms and was well-tolerated among children 5 to 12 years of age.

Social status and susceptibility to respiratory infections

Adults and children of lower socioeconomic status (SES) are at higher risk for a wide range of communicable infectious diseases, especially respiratory infections. Greater risk for infectious illness among people with lower SES is thought to be attributable to increased exposure to infectious agents and decreased host resistance to infection. We summarize three studies that examine the prospective association of several markers of social status (unemployment, perceived and observed social status) with host resistance to upper respiratory infections. Unemployment was associated with increased susceptibility to infection in adult humans. Lower social status in male monkeys was also associated with increased susceptibility, as was lower perceived social status in humans. The association of social status and susceptibility was accounted for primarily by increased risk in the lowest social status groups. However, further increases in social status were associated with further decreases in susceptibility in both monkeys and humans.

Influenza. The role of burden-of-illness research

Influenza is associated with a significant economic burden on both society and the individual, resulting in considerable healthcare costs and loss of productivity, as well as intangible costs such as suffering, grief and social disruption. The incidence and severity of influenza infection depend, at least in part, on the age and health status of the population. For example, the incidence of influenza is relatively high among children and young adults, but serious complications are much more likely to occur among the very young (< 1 year of age) and the elderly (> 65 years of age). School absenteeism tends to peak in the first half of a typical 6- to 8-week influenza epidemic, followed by workplace absenteeism in the latter half as school-aged children pass the infection to family members. Cost-of-illness studies are used by policy-makers to justify budgets and set priorities for prevention programmes, research and other expenditures. On the basis of German Sickness Fund data, recent estimates indicate that the cost of an influenza epidemic in that country is approximately 2 billion Deutschmarks (approximately $US1.4 billion). The bulk of these costs reflects indirect costs associated with lost productivity, a finding also noted in an earlier French cost-of-illness study of influenza. Thus, the main economic burden of influenza falls on infected individuals, their employers and their relatives. Methodology used in cost-of-illness studies can be quite variable. For example, two main approaches are used in measuring indirect costs (human capital and willingness to pay), although there is controversy as to which is the preferred method. Thus, investigators involved in cost-of-illness studies must be explicit regarding study methodology in order to allow for appropriate interpretation of study results by interested parties.

Influenza virus infections in infants

BACKGROUND

Universal immunization of children with live attenuated cold recombinant vaccine has been proposed. The renewed recommendation for maternal immunization with influenza vaccine should increase the amount of antibody transmitted to the infant and postpone the need for active immunization. This study examines the risk of influenza during the first year of life to provide information about the time to initiate active immunization.

METHODS

Infants followed from birth to 1 year of age in the Houston Family Study were monitored weekly for influenza virus infection. Serum specimens were tested for evidence of infection at 4-month intervals.

RESULTS

One-third of 209 infants were infected during the first year; most of the infections occurred during the second 6 months of life. Only 26 of 69 infections were detected before 6 months of age compared with 43 afterward. More striking was the concentration of serious illnesses in the latter half of the first year; 8 of 9 otitis media episodes and 9 of 11 lower respiratory tract illnesses occurred in the older infants.

CONCLUSIONS

The combination of increased maternal antibody titers that should result from influenza immunization and the lesser risk of influenza in the first 6 months of life allows initiation of active immunization of children after 6 months of age.

Comparison of rapid diagnostic techniques for respiratory syncytial and influenza A virus respiratory infections in young children

We performed virus isolation tests for respiratory viruses on combined nasal wash-throat swab specimens collected from infants and children with acute respiratory illnesses presenting to a hospital clinic during a 3-month period of concurrent epidemics of respiratory syncytial virus (RSV) and influenza A virus (Flu A) infections. Virus isolation results were used to assess the utility of commercially available rapid diagnostic kits for these two viruses. The kits employed direct immunofluorescence (IF) of cells (Imagen for RSV and Flu A), indirect IF of cells (Baxter Bartels Microscan), and enzyme immunoassay (EIA) (Becton Dickinson Directigen for RSV and Flu A and Abbott TestPack for RSV). All testing was completed on 81 specimens from 80 subjects. Of the 81 specimens, 53 (65%) yielded a virus: RSV, 28%; Flu A, 25%; rhinovirus, 6%; and enterovirus, cytomegalovirus, herpes simplex virus, and adenovirus, 2 to 4% each. Among the tests, Bartels Microscan and Directigen Flu-A exhibited the highest sensitivities (87 and 75%) and efficiencies (94 and 94%) for RSV and Flu A, respectively. All the tests exhibited high specificity. Thus, optimal detection of RSV and Flu A among infants and children who presented to a hospital clinic required two different detection methods (IF and enzyme immunoassay) and kits from two different companies (Baxter [Bartels Microscan] and Becton Dickinson [Directigen]).

Clinical manifestations and consequences of influenza

The spread of influenza virus through a community typically causes large increases in medical visits for febrile respiratory disease. Increased school absenteeism occurs early in the epidemic, and school children appear to be important for disseminating the virus. Industrial absenteeism, hospitalizations of adults and infants for pneumonia, and deaths due to pneumonia-influenza all tend to peak later in the epidemic. Although influenza infection rates are highest in persons of school age, hospitalizations and deaths occur primarily in infants and in the elderly, particularly among those with pulmonary, cardiovascular, or other debilitating disorders. Influenza viruses can be spread by aerosol or contact. The primary target cells are those of the respiratory epithelium. In healthy adults, the typical influenza syndrome includes fever, cough, and general aches for three to seven days, but lassitude, cough, and evidence of small-airways disease may persist for weeks. Laryngotracheobronchitis, pneumonia, and unexplained fever are prominent manifestations of influenza that lead to hospitalization of young children. Adults are more likely to have complications of bacterial pneumonia and worsening of chronic pulmonary disease or congestive heart failure. Less frequent complications include myositis, various neurologic disorders, and Reye's syndrome. These consequences of influenza clearly justify strenuous efforts at prevention and control.

Relation of serum antibody to glycoproteins of respiratory syncytial virus with immunity to infection in children

Immunity in relation to passively transferred maternal and naturally-induced serum antibody to the viral proteins was determined in 34 children who were followed from birth through three years of age for respiratory syncytial virus infection (RSV). Sera were tested by immunoglobulin class-specific enzyme-linked immunosorbent assay using the attachment and fusion proteins of the Long strain. The basis for immunity for maternal antibody in primary infection was assessed by a comparison of the distribution of antibody titers in a) 7 children who had an upper respiratory illness to 12 whose illness was accompanied by lower respiratory disease and of b) 13 children with an RSV-associated illness in the first 6 months of life who were age-matched as to month and approximate day of birth with 11 not infected in the same period. Infection induced immunity was evaluated by a comparison of antibody titers in 19 children who were reinfected with RSV in the year following their primary infection to 15 in whom reinfection was not documented. A statistical analysis of titers revealed that antibody to the fusion protein is an important correlate of immunity. In all three comparisons, the children with less RSV disease had significantly higher IgG anti-F titers prior to infection. No differences were observed between IgA anti-F or IgG and IgA anti-G titers.

Acquisition of serum antibodies to specific viral glycoproteins of parainfluenza virus 3 in children

A radioimmunoprecipitation assay was used to study antibody responses to parainfluenza virus 3 glycoproteins in human sera. The method was not only more sensitive than the neutralization test for the detection of antibody but also provided semiquantitative assessments of the antibody response to both glycoproteins in a single assay system. Anti-hemagglutinin-neuraminidase titers were consistently higher than anti-fusion levels in the same serum specimen. Thirteen children were monitored serologically and virologically from birth until 12 months or more after their primary infection with parainfluenza virus 3. At 1 to 3 months after infection, a significant increase in the level of antibody to the hemagglutinin-neuraminidase protein developed in 12 children; of these, 9 showed rises in the level of fusion protein. In 11 of the children, antibody titers continued to rise and the geometric mean titers to the hemagglutinin-neuraminidase protein was highest in sera collected 8 to 10 months after primary infection. Reinfection as the reason for these progressive increases in antibody levels could only be confirmed for four of the children. Three other children had reinfections after the 10-month sera were obtained; in each instance the only antibody responses were to the fusion protein.

Effects of social and family factors on viral respiratory infection and illness in the first year of life

A total of 131 infants were monitored from birth through the first year of life for respiratory viral infection and illness and evaluated for the relationship that these had to certain social and familial factors. The results showed no general patterns of association between viral infection and the study factors, but there were several significant individual associations. Excess influenza virus infection was found for black infants, infants with at least one sibling, and especially those with school age siblings. Rhinovirus infection rates were highest among girls attending daycare. In addition, significantly higher rates of lower respiratory disease (LRD) were seen in daycare infants and low socioeconomic infants and a definite trend to increasing amounts of LRD was seen with increasing family size. Protection from LRD seen in girls was apparently lost in daycare. No convincing differences for viral infection or respiratory illness were seen with parental smoking as an isolated factor.

Selected laboratory aspects of influenza surveillance

The importance of virologically documented infections in influenza surveillance is well recognized and has been reaffirmed in recent reviews. The large number of specimens tested in surveillance make efficiency and low cost of virologic methods important. Based on observations made by others and our work with reisolation of stored specimens we have used the continuous line tissue cultures MDCK and LLC-MK2 for virus isolation in large-scale influenza surveillance studies for three years. Both cell lines were equally successful in detecting influenza A viruses in 77 fresh, virus-positive specimens. However, during the influenza B outbreak of 1979--80, of 473 specimens positive in either or both tissue cultures, 54 were positive only in MDCK and just six in LLC-MK2 only. For parainfluenza viruses, LLC-MK2 was much superior to MDCK. The most promising alternative to tissue culture at this time, based on a review of the literature, appears to be enzyme immunoassay. Sensitivity sufficient for direct detection of viral antigen in routine specimens currently requires fluorescent or radioactive substrates. Identification of early virus growth in continuous cell line cultures by enzyme immunoassay is practical now and can be considered.