Cross sectional survey of influenza antibodies before and during the 2009 pandemic in Shenzhen, China

Abstract

Much information is available for the 2009 H1N1 influenza immunity response, but little is known about the antibody change in seasonal influenza before and during the novel influenza A pandemic. In this study, we conducted a cross-sectional serological survey of 4 types of major seasonal influenza in March and September 2009 on a full range of age groups, to investigate seasonal influenza immunity response before and during the outbreak of the sH1N1 influenza in Shenzhen - the largest migration city in China. We found that the 0-5 age group had an increased antibody level for all types of seasonal influenza during the pandemic compared to the pre-outbreak level, in contrast with almost all other age groups, in which the antibody level decreased. Also, distinct from the antibodies of A/H3N2, B/Yamagata and B/Victoria that decreased significantly during the 2009 H1N1 pandemic, the antibody of A/H1N1 showed no statistical difference from the pre-outbreak level. The results suggest that the antibodies against the 2009 sH1N1 cross-reacted with seasonal H1N1. Moreover, the 0-5 age group was under attack by both seasonal and 2009 H1N1 influenza during the pandemic, hence vaccination merely against a new strain of flu might not be enough to protect the youngest group.

Figure 1. The total number of ILI cases in each month of 2009 in Shenzhen.

In 2009, the peak of ILIs occurred in July 2009, sharply declined afterwards and formed a new wave in November. This may partially explain the significant drop in the three seasonal influenza antibody titer levels in September compared to March.

Figure 2. The proportion of each type of influenza in each month of 2009 in Shenzhen.

The 2009 H1N1 influenza pandemic reached its peak in September, and dominated all ILIs in October, according to a survey of 5,125 subjects. Meanwhile, the seasonal H1N1 incidences decreased to a very low level in September, but its antibody titers stayed at a high level. The H3N2 peaked in July but rapidly decreased in August and September. This suggests that the seasonal H1N1 influenza antibody might have been present in sH1N1-infected cases, and could have been associated with the 2009 H1N1 antibody. The seasonal H1N1 antibody was therefore persistent during the pandemic peak of the 2009 H1N1 but after the peak of its own antigen.