Seroprevalence of pandemic 2009 (H1N1) influenza A virus among schoolchildren and their parents in Tokyo, Japan

Abstract

Since its emergence, the 2009 pandemic H1N1 virus has spread rapidly throughout the world. Previously, we reported that most individuals born after 1920 do not have cross-reactive virus-neutralizing antibodies against pandemic (H1N1) 2009 virus, indicating that they were immunologically naïve to the pandemic virus prior to its emergence. This finding provided us with an excellent opportunity for a seroepidemiological investigation of the transmission mode of the pandemic virus in the community. To gain insight into its transmission within communities, we performed a serosurvey for pandemic virus infection with schoolchildren at an elementary school in Tokyo, Japan, and their parents. We observed a high prevalence of neutralizing antibodies to the pandemic virus in the children at this school, although the percentage of children positive for the neutralizing antibodies varied among classrooms. While a much lower prevalence was observed among parents, seropositivity of the parents correlated with that of their schoolchildren. Moreover, many adults appeared to have experienced asymptomatic infection with the pandemic virus. These data suggest that the pandemic virus was readily transmitted among schoolchildren in elementary schools and that it was also transmitted from schoolchildren to their parents.

Fig. 1.

Cases of pandemic H1N1 virus infection in Tokyo and serum collection dates. Information regarding the number of patients in Tokyo was reported by the Tokyo Metropolitan Institute of Public Health in Japan. Data were obtained from http://idsc.tokyo-eiken.go.jp/diseases/swine-flu/index.html.

Fig. 2.

Neutralization titers of sera against a 2009 pandemic H1N1 virus. The sera from groups 1 and 2 (schoolchildren and their parents at an elementary school, respectively; see details in Table 1) as well as from group 3 (no connection with the school; see details in Table 1) were subjected to virus neutralization assays with the pandemic H1N1 virus A/Osaka/364/09. (A) Each dot indicates the neutralizing antibody titer for each individual in groups 1 and 2 on 21 November 2009 (upper panel) and on 30 January 2010 (lower panel); (B) each dot indicates the neutralizing antibody titer for each individual in group 3. For a volunteer whose serum was obtained on multiple occasions, the latest result is shown. *, P < 0.001 by Fisher's exact test.

Fig. 3.

Seropositivity of sera collected in this study. Symptom status was obtained by interview. —, not applicable.

Fig. 4.

Seropositivity to the pandemic H1N1 virus among children in different classrooms. The virus neutralization activity in sera was tested as described in the Materials and Methods. For those who provided sera on several occasions at different time points, the latest serological data points were used. ^a, in Japan, children who are found to be influenza virus positive are forbidden to attend school. Before these children can return to school, a medical certificate from a physician confirming their recovery must be provided. We used this information to estimate the influenza morbidity in classrooms. *, P < 0.05 by Fisher's exact test indicates significant difference.

Fig. 5.

Comparison of seropositivity between volunteers who were vaccinated and those who were not vaccinated with a trivalent seasonal influenza vaccine in 2009. P < 0.05 by Pearson's test indicates a significant difference.

Fig. 6.

Estimated number of patients who consulted a physician for influenza-like illness. Weekly summary of patients showing influenza-like illness in Japan from 3 August 2009 to 13 December 2009 reported by the Ministry of Health, Labor, and Welfare (http://www.mhlw.go.jp/bunya/kenkou/kekkaku-kansenshou04/pdf/091225-01.pdf). These numbers were estimated from the data collected at sentinel clinics in Japan. y.o., years old.