Influenza Activity And Variation Of Pandemic H1N1 And Seasonal Influenza A Viruses In Shanghai

Influenza viruses have threatened human health for a long time as an important pathogen, in which influenza A viruses were considered as the most important monitoring targets because they can easily cause influenza epidemics in humans with wide host ranges and frequent genetic variation. Pandemic H1N1 which raised the global influenza pandemic in 2009 was a novel swine-origin influenza A virus. At present, epidemic surveillance and protection of high-risk population are the major measures to decrease the morbidity and mortality caused by influenza viruses. This study monitored and analyzed distribution and viral variation of influenza viruses, human immunity and viral evolutionary characteristics in recent years in Shanghai region, as the following parts:1. Distribution study of influenza subtype in Shanghai region, namely etiology surveillance from 2008 to 2011 in Shanghai region. Monitoring results showed that two peaks occured during 2008 to 2009:the first peak with seasonal H1N1 being dominated; the second peak with co-circulated seasonal H3N2 and H1N1. During 2009 to 2010, the monitoring results showed one peak occured in summer of 2009 with seasonal H3N2 being dominated, but pandemic H1N1 isolates were mainly identified since October, continued until 4th week of 2010. Influenza B viruses were first isolated in December of 2009 and became the dorminant strains since 5th week with the coincided attennated circulation of pandemic H1N1. But pandemic H1N1 strains were isolated in December again and by January of 2011 pandemic H1N1 became the epidemic strains in humans and raised a epidemic peak. The monitoring situation in Shanghai region was similar with the national monitoring situation.2. HAI antibody levels against influenza A viruses were detected in Shanghai population to understand the human immunity due to past infection and to forecast the flu epidemic trends. Pandemic H1N1 was a novel vius and had not been identified in humans, so we test human antibody levels against pandemic H1N1 before and after the pandemic to estimate the possible epidemic trends in humans. The results revealed that before the pandemic almost no immunity against pandemic H1N1 in humans, and through a epidemic period human immunity levels had increased but were still low so that pandemic H1N1 epidemic could not be fully inhibited in humans. But there has been a significient increase in antibody levels after vaccination and immune protection was provided by inoculating vaccine in humans. In addition, we tested antibody levels against seasonal H1N1, H3N2 subtype and avian flu H5 and H9 subtype in humans in Shanghai region in 2010. The results showed that humans have got immunity against seasonal influenza A viruses, and poultry contacted population had higher antibody levels against than the general population which explained that avian influenza viruses threatened to humans potentially.3. Gentic and antigenic analysis of influenza A viruses in Shanghai region. We chose some pandemic H1N1 strains isolated from 2010 to 2011 and sequenced HA,NA and PB2 segments. Phylogenetic trees indicated that no key mutations had appeated in HA genes of the early isolates in 2010 while some variations had occured in most isolates in winter and spring of 2011 compared with the early isolate A/California/07/2009(H1N1). Analysis of HA protein showed that some mutations had been found in antigenic determinant region in 2011 isoaltes and antigen drift had appeared. Mutations in this area can lower the human immunity to pandemic H1N1 strains. None changes had been detected related to the drug resistence to oseltamivir and zanamivir in NA protein. PB2 protein analysis showed that residue 627 and 701 were still Glutamic acid and A spartic acid, which were the same features of avian-origin influenza viruses. In addition, phylogenetic analysis of HA genes of seasonal influenza H1N1 viruses from 2005 to 2010 showed that epidemic strains of different years and seasons clustered in different branches and changed with the passing time and evolved to multiple directions under the pressure of human immune barrier. The 2010 isolates were similar with the vaccine strains A/Brisbane/59/2007(H1N1) which explained that the current vaccine can provide enough protection to humans.4. Co-infection study with pandemic H1N1 and influenza B viruses in MDCK cells.100TCID50 of paired pandemic H1N1 and influenza B isolates were inoculated in MDCK cells together, and then passaged continuously to compare the replication levels of pandemic H1N1 and influenza B virus under the co-infected condition. The results indicated that pandemic H1N1 viruses replicated more efficiently in MDCK cells than influenza B viruses, HA and PB2 segments of pandemic H1N1 and influenza B virus remained unchanged through all the passages and no mutation had been found in HA and PB2 region of both viruses. During the epidemic period the rates of detecting positive HI samples against pandemic H1N1 and influenza B virus were both low and pandemic H1N1 and influenza B virus could both raise epidemic in humans. The results can demonstrate that human natural protection did not contribute to the substitution of epidemic strains.