Incidence of influenza virus infections confirmed by serology in children and adult in a suburb community, northern China, 2018‐2019 influenza season

Abstract Background In mainland China, seasonal influenza disease burden at community level is unknown. The incidence rate of influenza virus infections in the community is difficult to determine due to the lack of well‐defined catchment populations of influenza‐like illness surveillance sentinel hospitals. Objectives We established a community‐based cohort to estimate incidence of seasonal influenza infections indicated by serology and protection conferred by antibody titers against influenza infections during 2018‐2019 influenza season in northern China. Methods We recruited participants in November 2018 and conducted follow‐up in May 2019 with collection of sera every survey. Seasonal influenza infections were indicated by a 4‐fold or greater increase of hemagglutination inhibition (HI) antibody between paired sera. Results Two hundred and three children 5‐17 years of age and 413 adults 18‐59 years of age were followed up and provided paired sera. The overall incidence of seasonal influenza infection and incidence of A(H3N2) infection in children (31% and 17%, respectively) were significantly higher than those in adults (21% and 10%, respectively). The incidences of A(H1N1)pdm09 infection in children and adults were both about 10%, while the incidences of B/Victoria and/Yamagata infection in children and adults were from 2% to 4%. HI titers of 1:40 against A(H1N1)pdm09 and A(H3N2) viruses were associated with 63% and 75% protection against infections with the two subtypes, respectively. Conclusions In the community, we identified considerable incidence of seasonal influenza infections. A HI titer of 1:40 could be sufficient to provide 50% protection against influenza A virus infections indicated by serology.


| INTRODUC TI ON
Pandemic and seasonal influenza viruses have been associated with a heavy burden on morbidity and mortality worldwide. 1,2 Influenza surveillance, which provides real-time information to inform prevention and control policy, focused on cases seeking medical care. There are some barriers to assess influenza disease burden at community level using existing surveillance platforms.
The data derived from surveillance could not capture infections associated with mild symptoms or not requiring medical attention.
Information on the community burden of influenza is a key to informing control. Some community-based cohort studies have been conducted to provide important insight into the disease burden and transmission behavior of influenza. [3][4][5][6][7][8][9] These studies largely took place in the United States between 1948 and 2013, [3][4][5][6] and recently, similar studies were undertaken in England, Hong Kong, Vietnam, etc. [7][8][9] Additionally, more cohort studies were conducted to estimate disease burden of influenza during 2009-10 pandemic influenza. 10,11 Although hospital-based influenza-like illness (ILI) sentinel surveillance has been established in mainland China to monitor influenza activity, 12,13 the incidence rate of influenza virus infections in the community is difficult to determine due to the lack of well-defined catchment populations of surveillance sentinel hospitals. Very few data are available about influenza disease burden at community level in mainland China. Only several cross-sectional serological surveys were conducted to determine the prevalence of antibodies to A(H1N1)pdm09 during the 2009 influenza pandemic. 14,15 Therefore, we established a community-based cohort to qualify incidence of seasonal influenza infections confirmed by serology after experiencing a wintertime influenza season of 2018-2019 in a semi-rural community of northern China, and to estimate the protection conferred by different antibody titers against seasonal influenza infections.

| Recruitment and follow-up
Our cohort study was conducted in three villages of suburb area of Changzhi City, Shanxi Province, that is located in northern China. Local residents aged 5-59 years were recruited by doctors and nurses working at village-level health clinics to participate in our study via face-to-face invitation or the invitation by phone call from mid-November 2018 to late-December 2019 (pre-season survey). When the local residents agreed to participate, they were asked to complete a questionnaire including demographic, underlying medical conditions, etc. Serum samples were collected from the participants by trained nurses. The participants were followed up from mid-May to late-May 2019 (post-season survey). During the post-season survey, a short questionnaire including influenza vaccination information in 2018-2019 influenza season and collection of sera was completed.

| Ethics
Proxy written consent from parents or legal guardians was obtained for participants ≤17 years of age, with additional written assent from those aged 8-17 years of age. Written consent was obtained from all adult participants. The study protocol was approved by the Institutional Review Board of Shanxi Provincial Center for Disease Control and Prevention.

| Laboratory test
The paired sera were tested in parallel by hemagglutination in-

| Statistical analysis
Hemagglutination inhibition antibody titers of paired sera were compared to determine serologic evidence of infections of influenza viruses, indicated by a 4-fold or greater increase in antibody titers between pre-season and post-season sera with an antibody titer of 1:40 or more in post-season sera. We used Poisson regression model to estimate the incidence rates of influenza virus infections indicated by serology in children aged 5-17 years and adults aged 18-59 years, respectively. We used a logistic mixed-effect regression model to estimate the association of influenza infections indicated by serology with age-groups and other factors. We used logistic regression to estimate the relationship between protection against influenza infections and antibody titers. 18 The protection rate at different antibody titer was calculated as the odd ratio (OR) reduction compared with the OR at an HI titer <1:10. We compared medians of pre-season HI antibody titers between children and adults, between participants infected and not infected with seasonal influenza viruses, using Wilcoxon signed-rank test after log transformation.

| RE SULTS
Our study recruited 228 children aged 5-17 years and 457 adults  Table 1). 31% of participants followed up were male. 10% of participants followed up had tertiary education or above. None of the participants received seasonal influenza vaccine during the 2018-2019 influenza season.
Of 616 participants who were followed up, 426 participants clustered in 192 households. There were significantly higher proportions of children 5-17 years of age, students, and male participants among the participants from household clusters than among those not from household clusters ( Table 2).

| D ISCUSS I ON
We established a cohort of children and adults in an unvaccinated community and collected their paired sera before and after 2018-2019 influenza season to enable inference on incidence of seasonal influenza infections. Our study showed that seasonal influenza infections were common in unvaccinated children and adults. We   As observed in the previous prospective studies, our study identified multiple infections with different influenza subtype/lineage in the same season, and they could occur in children and adults. 8,24 So far, the scientific significance of the phenomenon is unclear.
We observed that pre-season HI antibody titers against four vaccine strains in children were all higher than those in adults,   Finally, due to limited research resource, we cannot undertake surveillance for clinical illness of subjects. So we cannot estimate the protection rate of HI antibody titers against clinical illness.
In conclusion, we found substantial incidence of seasonal influenza infections in the unvaccinated community during the 2018-2019 influenza season. The school-aged children had higher overall incidence of seasonal influenza infections and higher incidence of A(H3N2) infection than adults aged 18-59 years. We demonstrated that a HI titer of 1:40 was sufficient to provide 50% protection against influenza A infections indicated by serology.

CO N FLI C T O F I NTE R E S T
The authors report no other potential competing interests.

DATA AVA I L A B I L I T Y S TAT E M E N T
The datasets in our study are available from the first author and correspondence author.