Simulation studies of influenza epidemics: assessment of parameter estimation and sensitivity

The influenza simulation model of Elveback et al is used to evaluate the accuracy of the maximum likelihood procedure of Longini et al for estimating the secondary attack rate in households. The sample population from the Tecumseh Respiratory Illness Study is mapped into the simulation model and simulations are carried out over a range of parameter values and conditions, some of which were derived from influenza seasons in Tecumseh and from the Seattle Flu Study for the years 1975-1980. The estimation procedure is found to be quite robust for parameter values preset within appropriate limits for influenza. However, a significant difference is found between the preset and estimated household contact parameter for epidemics of medium and high intensity when the preset value is zero. Incremental increases in the household contact parameter are shown to produce marked increases in the overall infection attack rate demonstrating that household spread is an important link in maintaining infection in other mixing groups such as schools, preschool groups and neighbourhood clusters of households.

Transplacental passage of influenza A/Bangkok (H3N2) mimicking amniotic fluid infection syndrome

Influenza virus infection complicating pregnancy remains a matter of concern because of the frequency of these infections and possible associated increased maternal and perinatal risks. Influenza A/Bangkok (H3N2) infection occurred in a gravid woman in association with fever, chills, and uterine tenderness and contractions together with maternal and fetal tachycardia. Initial evaluations led to consideration of amniotic fluid infection syndrome with planned termination of the pregnancy. Examination of amniotic fluid obtained by amniocentesis showed neither microorganisms nor neutrophils, and close monitoring and supportive measures were continued. Amniotic fluid and maternal nasopharyngeal washing both yielded influenza A/Bangkok (H3N2). Mother and fetus were closely monitored until term. Cord blood samples demonstrated hemagglutination inhibition and complement fixation antibody titers of 1:32 and greater than 1:512, respectively. Cord blood IgM and IgA hemagglutination inhibition antibody concentrations were 6,400 and 3,200 micrograms/ml, respectively, consistent with transplacental infection. This is the first confirmation of transplacental influenza infection. This generally self-limited viral infection may mimic the amniotic fluid infection syndrome and put mother and fetus at risk for potentially harmful untimely delivery.

Age and secular distributions of virus-proven influenza patients in successive epidemics 1961-1976 in Cirencester: epidemiological significance discussed

A general practice population of around 3900, under continuous clinical and laboratory surveillance, experienced 20 outbreaks of influenza between March 1960 and March 1976. Four epidemics were caused by subtype H2N2 type A viruses, seven by subtype H3N2 type A viruses and nine outbreaks by type B viruses. The age of every person proved virologically to have influenza is related to the age structure of the community and to the phase of the epidemic in which the virus-positive specimens were collected. Children 0-15 years old suffered a higher incidence rate than adults 16-90+. Pre-school children 0-4 suffered the highest rate of infection by viruses of both influenza A subtypes, whereas older schoolchildren 10-15 suffered the highest rate of type B infections. Despite these high incidence rates neither pre-school nor schoolchildren appear to have been the major disseminators of any of these influenza viruses in the community. Adults of all ages suffered a high rate of infection even into extreme old age, and the indiscriminate age distribution among adults was sustained in the successive epidemics. Such age-patterns are not those caused by a highly infectious immunizing virus surviving by means of direct transmissions from the sick, whose prompt development of the disease continues endless chains of transmissions. An alternative epidemic mechanism--whereby the virus does not spread from the sick but becomes latent in them, reactivating seasonally so that they later infect their companions--would produce age patterns similar to those recorded here for influenza patients. The suggested mechanism is illustrated by a simple conceptual model and the influenzal age patterns are discussed in relation to the recycling of influenza A subtypes.

Influenza A prophylaxis with amantadine in a boarding school

In a boarding school for boys, where routine influenza vaccination is carried out annually, 267 boys were given amantadine (100 mg daily) and 269 received no specific treatment during an influenza A (H1N1) outbreak. 3 boys receiving amantadine and 29 boys receiving no medication had laboratory-proven influenza A. There are circumstances when the prophylactic use of amantadine may be justified for the control of influenza A outbreaks in boarding schools and other institutions where high attack-rates are experienced.

Influenza B virus infections in the community and the family. The epidemics of 1976-1977 and 1979-1980 in Houston, Texas

Influenza B virus epidemics occurred in Houston, Texas, in 1976-1977 and 1979-1980. Among families with young children followed longitudinally in the Houston Family Study, 112 infections were detected during 511 person-years of observation. The infection rates for the two epidemics were similar--24 per cent and 20 per cent--although the two epidemics differed greatly in the community. The first epidemic was much more intense with a mid-winter peak that produced school absentee rates above 12 per cent for four consecutive weeks. The indolent epidemic of 1979-1980 smoldered from late September to mid-April with a peak during the second week of March for which school absenteeism did not exceed 8 per cent. In the Houston Family Study population, the combined infection rate for the two outbreaks was highest at 35 per 100 person-years for school children aged 6-19 years. Preschool children aged 7 months-5 years and adults had infection rates of 31 and 16 per 100 person-years, respectively. Preexisting neutralizing antibody titers greater than or equal to 3.5 log2 protected against influenza B infection and illness. Preschool children above 6 months of age, school age children, and parents introduced infection into the family at rates of 15, 15, and 9 per 100 person-years, respectively. Three second introductions were observed. The secondary infection rate was highest among school aged children at 61 per 100 persons at risk.

[Influence of influenza epidemics on the urban population]

The results of studies carried out in 1974-1979 according to the programme of epidemiological survey of influenza among the population of Houston (USA) developed by the Research Center for Influenza at the Baylor Medical College are reviewed. The data of selective virological and epidemiological surveys of the population were used for comparative analysis of several epidemic outbreaks caused by different antigenic variants of influenza virus. The analysis has been aimed at the detection of "high risk" groups of the population, investigation of the effect of influenza on the health of the population, determination of the factors conducive to virus spread, development of methods for the evaluation of the efficacy of influenza control measures, as well as determination of the earliest signs of activity of influenza viruses among the population and search for the best method of prediction of an epidemic virus for the following season.

[Procedures for estimating transmission parameters from influenza epidemics: use of serological data]

A maximum likelihood procedure is given for estimating household and community transmission parameters from observed influenza infection data. The mathematical model used does not require the specification of infection onset times and, therefore, can be used with serological data which detect asymptomatic infections. Infection data was derived by serology and virus isolation from the Tecumseh Respiratory Illness Study and the Seattle Flu Study for the years 1975-1979. Influenza A (H1N1), A (H3N2), and B viruses were found to be in descending order both in terms of ease of spread in the household and intensity of the epidemic in the community except when two strains co-circulate. Children are found to be the main introducers of influenza into households.

[Role of passerine birds in the ecology of influenza viruses]

A total of 267 passerine birds distributed among 37 species were netted during spring 1980 and summer 1981 in the Laurentian and Montreal areas. All the cloacal swabs collected at that time wer free of influenza viruses. Three and five days after oral administration of avian or human influenza A virus strains, 108 isolates were obtained from 42 of 134 passerine birds. Positive samples were recovered mainly from the respiratory and the digestive tract and also from liver. Spleen and kidneys. Viral replication is cells from trachea, lungs, gizzard and caecum was detected by indirect immunofluorescence using a monoclonal antibody to influenza A virus nucleoprotein. Viral transmission from inoculated to non inoculated birds placed in the same cages was not observed. On the other hand a similar experimental inoculation of young mallard ducks showed that extensive viral transmission occurred from inoculated to non inoculated ducklings and that infection was found exclusively in the digestive tract. Furthermore viruses were detected in samples of drinking water from all cages containing infected ducks. Passerine birds do not represent an important reservoir of influenza viruses but might contribute to the formation and spreading of recombinants potentially pathogenic for man and animals.

The diffusion of influenza in sub-Saharan Africa during the 1918-1919 pandemic

The focus of this study is the spread of influenza in Africa south of the Sahara during the pandemic of 1918-1919. Most known types of diffusion pattern, i.e. radial, wave-like and linear, have been identified; however, the disease spread to rapidly that four particularly devastating linear patterns stand out. One of the most incredible aspects of the regularity in epidemic velocity along these pathways was that the colonial transportation network was of fairly recent origin. Given modern transportation linkages, the ever-present danger of the resurgence of the agent or agents that caused the pandemic could result in a much greater disaster in Africa and other Third World areas if unabated by effective inoculation programs.

Estimating household and community transmission parameters for influenza

A maximum likelihood procedure is given for estimating household and community transmission parameters from observed influenza infection data. The estimator for the household transmission probability is an improvement over the classical secondary attack rate calculations because it factors out community-acquired infections from true secondary infections. The mathematical model used does not require the specification of infection onset times and, therefore, can be used with serologic data which detect asymptomatic infections. Infection data were derived by serology and virus isolation from the Tecumseh Respiratory Illness Study and the Seattle Flu Study for the years 1975-1979. Included were seasons of influenza B and influenza A subtypes H1N1 and H3N2. The transmission characteristics of influenza B and influenza A(H3N2) and A(H1N1) outbreaks during this period are compared. Influenza A(H1N1), A(H3N2) and influenza B are found to be in descending order both in terms of ease of spread in the household and intensity of the epidemic in the community. Children are found to be the main introducers of influenza into households. the degree of estimation error from the misclassification of infected and susceptible individuals is illustrated with a stochastic simulation model. This model simulates the expected number of detected infections at different levels of sensitivity and specificity for the serologic tests used. Other sources of estimation error, such as deviation from the model assumption of uniform community exposure and the possible presence of superspreaders, are also discussed.

Household and community transmission parameters from final distributions of infections in households

A model is devised for the distribution of the total number of cases in households from a homogeneous community. In the model, community-acquired infection serves as a source of initial infection within households as well as of possible further cases. In addition, infected household members can infect others in the household. Maximum likelihood procedures for the model parameters are given. The model is fitted to symptom data on influenza and the common cold. Influenza seems to spread more easily in the community than within the household, while the opposite may be the case for the common cold. The model, which does not require specification of the time of onset of infection for individuals, can be fitted to serological data; this would provide a more accurate measure of household infection than the symptom data used.

Patterns of shedding of myxoviruses and paramyxoviruses in children

In the Houston Family Study, young children were cultured for virus weekly or biweekly and during acute respiratory illnesses. The interval between the onset of illness and positive culture was examined for 179 infections during 1975-1979. In week 1 after onset, 73%, 73%, and 66% of cultures were positive for influenza A virus, respiratory syncytial virus (RSV), and parainfluenza virus type 3, respectively. Pooled data from influenza B virus infections in 1977 and 1980 showed that 73% of cultures were positive in week 1. Influenza A virus in week 2 or RSV in weeks 2 and 3 was isolated from very few children. However, 37% of cultures were positive for influenza B virus during week 2, and 17% of cultures were still positive for parainfluenza virus type 3 during week 3. Shedding of parainfluenza virus type 3 on days 29-38 was also observed. Parainfluenza virus type 3, RSV, and influenza A virus were isolated up to six days before the onset of illness.

A five-year study of influenza in families. Joint Public Health Laboratory Service/Royal College of General Practitioners working group

A five year collaborative study of influenza in volunteer families from 1973-78 covered a period in which there were outbreaks every year but no major epidemics of influenza. Volunteers over the age of 15 years were bled before and after each of the five winters, and virus isolation was attempted from as many as possible when they reported episodes of illness. Children under 15 in the volunteer families were also swabbed when they were ill. Although most families experienced one or more attacks by influenza viruses, there was little transmission within families.

[Infections causes of congenital abnormalities]

Revisão sumária das causas infecciosas das anomalias congênitas englobando as malformações, que correspondem a estruturas anormais orgânicas ou tissulares decorrentes de erros primários de morfogenese embrionaria e deformações, que se instalam no período fetal da vida intrauterina, correspondentes a alterações de forma e estrutura de órgãos primitivamente bem constituidos.

The role of season in the epidemiology of influenza

Four types of observations have been used to illustrate the seasonal characteristics of epidemic influenza: (1) The experience of a small population during 28 consecutive years, 1946-74, (2) world influenza outbreaks 1964-75 reported to the World Health Organization, (3) the experience of two widely separated localities at about the same latitude, 1969-74, and (4) the experience of two places at latitudes 30 degrees + on opposite sides of the Equator, 1968-74. The following tendencies are shown. (1) Outbreaks of influenza even in the small community came at approximately the same season almost every year. (2) Outbreaks are globally ubiquitous and epidemic loci move smoothly to and fro across the surface of the earth almost every year in a sinuous curve that runs parallel with the 'midsummer' curve of vertical solar radiation, but lags about six months behind it. Such findings exclude the mediation of seasonal control by any agencies of local distribution, and suggest a direct effect of variations in some component of solar radiation on virus or human host. (3) Antigenic variations in influenza A virus tended to have the same seasonal characteristics as epidemicity. This suggests that epidemicity and virus variation are two facets of one seasonally controlled process. None of these seasonal characteristics can be explained by the current concept of influenzal epidemiology. A new hypothesis recently proposed and recapitulated in the Appendix offers a possible explanation. The primary agency mediating seasonal control remains unidentified.

Evidence for the natural transmission of influenza A virus from wild ducts to swine and its potential importance for man

In 1979, epidemics of influenza occurred in pigs in Belgium from which were isolated strains of influenza A (Hsw1N1) virus antigenically closely related to Hsw1N1 strains previously isolated from ducks in North America and the Federal Republic of Germany. This finding is considered as the first supportive evidence that an influenza A virus in an avian species might have been transmitted to mammals.

The ecology of influenza viruses: a WHO memorandum

Influenza A viruses continue to be isolated from man, pigs, horses, and a wide range of avian species, especially ducks. The recent isolation of an influenza A virus from seals has added an additional mammal to the list of natural hosts for these viruses. In contrast, influenza B viruses have been isolated only from man.The haemagglutinin of a virus isolated from gulls in the United States of America could not be identified with reference antisera and may constitute a new haemagglutinin subtype.Studies in swine suggest that H1N1 viruses related to A/New Jersey/8/76 continue to circulate in some countries of the world, and that the H3N2 and H1N1 viruses can be transmitted from man to pigs. The isolation from pigs of an H1N1 virus that is antigenically similar to avian H1N1 isolates and the isolation of "classical" swine influenza virus from a piglet in Czechoslovakia are of particular interest and raise the question of the source of the viruses.Preliminary studies on equine influenza viruses (H3N8) (Heq2Neq2) from horses in Europe suggest that significant antigenic drift has occurred in viruses of this subtype since the prototype was identified in 1963.The role of influenza viruses from lower animals and birds in the genesis of new human strains is unknown, and the genes responsible for host range and virulence have not yet been identified. The availability of techniques for analysis of all the RNAs and the gene products will permit further studies on these important questions.