Epidemiology of measles in Taiwan: dynamics of transmission and timeliness of reporting during an epidemic in 1988-9

Near real-time surveillance of the SARS-CoV-2 epidemic with incomplete data

When responding to infectious disease outbreaks, rapid and accurate estimation of the epidemic trajectory is critical. However, two common data collection problems affect the reliability of the epidemiological data in real time: missing information on the time of first symptoms, and retrospective revision of historical information, including right censoring. Here, we propose an approach to construct epidemic curves in near real time that addresses these two challenges by 1) imputation of dates of symptom onset for reported cases using a dynamically-estimated "backward" reporting delay conditional distribution, and 2) adjustment for right censoring using the NobBS software package to nowcast cases by date of symptom onset. This process allows us to obtain an approximation of the time-varying reproduction number (Rt) in real time. We apply this approach to characterize the early SARS-CoV-2 outbreak in two Spanish regions between March and April 2020. We evaluate how these real-time estimates compare with more complete epidemiological data that became available later. We explore the impact of the different assumptions on the estimates, and compare our estimates with those obtained from commonly used surveillance approaches. Our framework can help improve accuracy, quantify uncertainty, and evaluate frequently unstated assumptions when recovering the epidemic curves from limited data obtained from public health systems in other locations.

Near real-time surveillance of the SARS-CoV-2 epidemic with incomplete data

Designing public health responses to outbreaks requires close monitoring of population-level health indicators in real-time. Thus, an accurate estimation of the epidemic curve is critical. We propose an approach to reconstruct epidemic curves in near real time. We apply this approach to characterize the early SARS-CoV-2 outbreak in two Spanish regions between March and April 2020. We address two data collection problems that affected the reliability of the available real-time epidemiological data, namely, the frequent missing information documenting when a patient first experienced symptoms, and the frequent retrospective revision of historical information (including right censoring). This is done by using a novel back-calculating procedure based on imputing patients' dates of symptom onset from reported cases, according to a dynamically-estimated "backward" reporting delay conditional distribution, and adjusting for right censoring using an existing package, NobBS , to estimate in real time (nowcast) cases by date of symptom onset. This process allows us to obtain an approximation of the time-varying reproduction number ( R t ) in real-time. At each step, we evaluate how different assumptions affect the recovered epidemiological events and compare the proposed approach to the alternative procedure of merely using curves of case counts, by report day, to characterize the time-evolution of the outbreak. Finally, we assess how these real-time estimates compare with subsequently documented epidemiological information that is considered more reliable and complete that became available later in time. Our approach may help improve accuracy, quantify uncertainty, and evaluate frequently unstated assumptions when recovering the epidemic curves from limited data obtained from public health surveillance systems in other locations.

Measles immunity gaps in an era of high vaccination coverage: A serology study from Taiwan

BACKGROUND

Routine vaccination for children drastically reduces measles cases. Nonetheless, the global resurgence of measles since 2018 highlights the need for evaluating vaccine-induced immunity in an era of high vaccination coverage, especially in international travelers. This study aims to identify high-risk persons via analyzing the impact of age and different vaccination schedules on seroprevalence of measles in a country with high vaccination coverage, attempting to provide suggestions regarding pre-travel measles vaccination in highly immunized countries.

METHOD

We retrospectively collected serological results of measles from Taiwanese travelers during 2008-2017 and middle-aged subjects from community during 2007-2009. Participants were classified by age groups and cohort based on the national immunization program: cohort I (pre-mass vaccination), cohort II (suboptimal measles vaccination), and cohort III (2-dose MMR in childhood). The effects of age group on seropositivity was shown by odds ratio (OR) with 95% confidence interval (CI) in a logistic regression model.

RESULTS

The overall measles seroprevalence was 74.5% (N = 1096, mean age = 31.7 ± 12.3 years), with adolescents having the lowest seropositivity (65.9%) and participants ≥50 years having the highest (89.4%). Seropositivity was significantly higher in cohort I (pre-mass vaccination) compared with the other 2 cohorts (P < 0.0001). The effect of aging on seropositivity was only significant in cohort II (OR = 1.84, 95% CI = 1.13-2.99, P = 0.014 for 30-39 years vs. 20-29 years).

CONCLUSIONS

Waning immunity to measles in adolescents and young adults is a dominant issue in highly vaccinated cohorts. Enhancing pre-travel vaccination against measles can effectively fill the immunity gaps in highly immunized countries.

Timely measles surveillance in the Republic of Korea, 2002-2009: impact of sentinel laboratory surveillance

In order to sustain the elimination of measles, timely reporting is important. The surveillance data in Korea from 2002-2009 was analyzed to determine the effect of sentinel laboratory surveillance, which was introduced in 2006, on the timeliness of measles reporting. The data were stratified by two surveillance periods, (A) before and (B) after 2006, and by cases confirmed clinically and cases confirmed by laboratory measures. During Period A, 113 suspected cases were reported, and 241 during Period B. There was no difference in the proportion of timely reporting among cases confirmed clinically between the two periods, whereas the proportion of cases confirmed by laboratory measures has increased. The mean notification interval in cases confirmed by laboratory measures was shortened from 39 to 16 days. In Korea, sentinel laboratory surveillance has enhanced earlier detection of suspected cases that had not been reported, improving the timeliness of measles surveillance. Adopting this new method may improve the timely collection of cases in other countries.

Current status of measles in the Republic of Korea: an overview of case-based and seroepidemiological surveillance scheme

Following the Five Year Measles Elimination Program, measles has been declared eliminated from the Republic of Korea since 2006. However, there remain challenges related to the surveillance of measles in the postelimination phase. Even though the routine surveillance system has revealed a gradual decrease in the number of reported cases since 2002, 4 resurgences have occurred, notably due to outbreaks. Because vaccine-modified measles is becoming widespread due to high vaccination coverage, conducting laboratory confirmation in each case becomes important. Moreover, susceptible individuals with measles have been identified through seroprevalence studies. Lastly, the efforts to improve the timeliness of measles reporting have led to the establishment of an active laboratory-based surveillance network, which has shortened the interval between diagnosis and notification. In these circumstances, searching for more sensitive and effective surveillance measures is important for maintaining the elimination status and preventing future outbreaks of measles in Korea.

Changes in measles seroepidemiology of healthcare workers in southern Taiwan

To assess the measles seroepidemiology of healthcare workers (HCWs), from 2004 to 2009 all newly recruited staff at a university hospital in Taiwan, were tested for specific immunoglobulin G antibodies for measles virus using enzyme-linked immunosorbent assays. A total of 1584 HCWs were included. The positivity rate of measles antibody for physicians and nurses was about 90%. The positivity of measles antibody in different age groups was 78·1%, 93·9% and 94·2% for 20-29, 30-39 and ≥40 years age groups, respectively. Equivocal results were more frequent in the 20-29 (17·1%) years age group. A lack of antibody against measles was reported for 66 (4·2%) of those tested. Medical student (8·3%) and housekeeping personnel (5·3%) had the greatest lack of antibodies. This study demonstrates the waning measles immunity in younger HCWs in Taiwan, which poses the potential risk of nosocomial transmission of measles. Periodic surveillance and revaccination of susceptible HCWs is recommended.

Modelling vaccination programmes against measles in Taiwan

Vaccination has proved a powerful defence against measles. We reappraise measles seroepidemiological data in Taiwan from 1974 to 2004 having robust age-stratified serological information on exposure and immunity to quantitatively characterize measles vaccination programmes. We dynamically model measles seroepidemiology to estimate age-dependent intensity of infection associated with the effects of different contact patterns on pre- and post-vaccination. The WAIFM (who acquires infection from whom) contact matrix is employed to describe the transmission between and within each age group. A deterministic SEIR (susceptible-exposed-infected-recovery) model is used to capture subpopulation dynamics. Our study shows that mass regional or nationwide vaccination programmes could greatly reduce the potential for a major measles epidemic and have strong direct effects on the potential impact of childhood vaccination. We parameterize a predictive model that should reduce the socio-economic costs of measles surveillance in Taiwan and thereby encourage its continuance, especially for preschool children.

Measles IgG seroprevalence prior to mass vaccination in Taiwan

OBJECTIVES

Measles vaccine was licensed for use in Taiwan in 1968 and mass vaccination has been implemented since 1978. Serological surveys were conducted to characterize measles seroepidemiology prior to mass vaccination.

METHODS

Measles IgG was quantitatively measured in 1564 serum samples collected from four populations (nationwide, urban, rural, and mountain areas) during 1974-80 with a commercial EIA kit. The nationwide samples covered age groups from 3 months to 19 years, but the other three samples only covered school-aged children. The cut-off value of seropositivity was set at 100 mIU/ml.

RESULTS

In the nationwide samples, measles IgG seroprevalence decreased from 50% at 3-5 months to 13% at 6-11 months, then increased rapidly to 55% at 1-1.9 years and reached 98% at 7-7.9 years of age; this was maintained to young adulthood. In all four samples, seroprevalence also reached >97% in individuals >10 years of age. Semiquantitative analysis found that 24%(10/42) of 1-1.9-year-old children but only 4%(1/24) of 15-19-year-old adolescents had low level measles IgG (100-399 mIU/ml). In addition, the distribution of measles IgG level in 1-1.9-year-old children had two peaks at 200-399 and 1600-3199 mIU/ml, whereas only one peak at 1600-3199 mIU/ml had been found in 15-19-year-old adolescents. The median age of measles infection (i.e. 50% seroprevalence) in Taiwan was 1.4 and 2.1 years, respectively, before and after excluding individuals with measles IgG 100-399 mIU/ml that may have resulted from vaccination.

CONCLUSIONS

Measles seroepidemiology in Taiwan in the 1970s seemed to be more similar to that in a developing country rather than in an industrialized country, and there was a low median age of infection.

Measles seroepidemiology and decay rate of vaccine-induced measles IgG titers in Taiwan, 1995-1997

Community-based seroepidemiologic studies were conducted to monitor the effectiveness of measles immunization programmes and to estimate the decay rate of vaccine-induced measles IgG titres. Sera collected from a mountain (792 sera), rural (875 sera) and urban (894 sera) populations in 1995-1997 were available. Measles IgG was quantified using a commercial EIA kit. Measles IgG seroprevalence and geometric mean titre (GMT) were calculated by setting the cut-off titre at 50 mIU/ml. The decay rate of measles IgG titres was estimated by assuming that the measles IgG titres, without exposing to wild measles virus, decay exponentially and constantly after 1 year post vaccination. The half-life of measles IgG titres was calculated from the corresponding decay rate. Measles IgG seroprevalences in these three populations have reached >95% in school children (7-18 years old) and >98% in young adults (19-25 years old) but varied from 87 to 96% in pre-school children (4-6 years old). Two-dose vaccinees, comparing with 1-dose vaccinees, had a significantly higher seroprevalence (98 versus 92%, P<0.01) and a slightly longer half-life of measles IgG titres (61 versus 27 months, P=0.08) but the measles IgG GMT in the two groups did not differ significantly (675 versus 618 mIU/ml, P=0.78).

Protective titres of measles neutralising antibody

A nation-wide measles outbreak occurred in 1988 in Taiwan. A retrospective cohort study was conducted to define the protective titre of measles neutralising (NT) antibody. Paired sera collected in 1987 and 1988 were available from 190 individuals born in 1984 who had participated in an annual hepatitis B immunisation follow-up from 1986 to 1991. Measles NT titres were quantified using a standardised neutralisation enzyme immunoassay. Measles infection was defined as a >/=4-fold rise in NT titre or seroconversion between paired sera. Symptomatic measles infection was ascribed to individuals who had measles infection and who reported measles-like symptoms between 1987 and 1988. Results demonstrated a dose-response relationship between pre-exposure NT titres and protection against measles infection. 47 of 48 individuals with measles infection in 1988 had pre-exposure NT titres </=1,017 mIU/ml; all 12 symptomatic cases had pre-exposure NT titres </=434 mIU/ml. Eleven vaccinees had pre-exposure NT titres <50 mIU/ml, of whom nine developed asymptomatic infection. The study suggests that measles NT titres >1, 000 mIU/ml may prevent measles infection and NT titres >500 mIU/ml may prevent symptomatic infection but vaccinees with undetectable or low NT titres may not necessarily be susceptible to symptomatic infection.

Post mass-immunization measles outbreak in Taoyuan County, Taiwan: dynamics of transmission, vaccine effectiveness, and herd immunity

OBJECTIVES

Analysis of national surveillance data and a seroepidemiologic investigation were conducted to elucidate the causes and epidemiologic characteristics of a measles outbreak in Taoyuan, Taiwan, 1994.

METHODS

Measles cases were identified through a national surveillance system. Reported cases and their physician or school nurses were interviewed to trace additional suspect cases and were sampled for serologic diagnosis. Measles-specific IgG and IgM were assayed. A confirmed case was defined as being positive for measles IgM test but not having received measles vaccination within the previous 3 months.

RESULTS

The outbreak began in Taoyuan City in December 1993 and continued to spread in primary schools and kindergartens, but caused only sporadic cases in neighboring towns. Among 42 confirmed cases, 15 (38%) were primary school children and 16 (38%) were kindergarten children. Among 24 confirmed cases with a vaccination record, 7 had one dose of vaccination, 4 had two doses of vaccination, and 13 (54%) were unvaccinated. The overall measles susceptible proportion at a kindergarten before the outbreak was 8.1% (17/209) and the overall measles cumulative incidence among susceptibles was 0.65 (11/17).

CONCLUSIONS

A measles vaccination coverage of 82% with the first dose at 9 months of age and 63% with the second dose (measles, mumps, and rubella) at 15 months was inadequate to block measles virus circulation in Taoyuan City in 1994. The city center, with a growing population, represents a high risk as an epicenter for measles outbreaks. Measles outbreaks may occur in a school population with 92% herd immunity.

[Measles outbreaks: considerations apropos of the outbreaks investigated in Catalonia (1990-1995)]

OBJECTIVES

To determine the factors associated with outbreak size in Catalonia.

METHODS

Outbreaks over the period 1990-95 were investigated. A comparison was run between outbreaks of more than 6 cases with respect to outbreaks of 2-6 cases in function of the following variables: median age of cases, transmission setting and year of presentation. Multiple logistic regression was used to asses the independent effect of each variable, with the adjusted odds ratio (ORa) and their 95% confidence interval (CI) being obtained.

RESULTS

In all, 45 outbreaks were studied, with 595 cases of measles 39.5% of outbreaks occurred in the 6-10 age group. Outbreaks of more than 6 cases had a higher likelihood of appearing in a school setting (ORa = 4.3, CI 95% 1.0-17.6); but there were no association with age (ORa = 2.1, CI 95% 0.5-9.2) or occurring prior to 1994 (ORa = 0.9, CI 95% 0.2-3.6).

CONCLUSIONS

School setting determines the outbreak size of measles and demands to apply specific preventive strategies.