14th ICID - Poster Abstracts - International Society for Infectious ...

When citing these abstracts please use the following reference:
Author(s) of abstract. Title of abstract [abstract]. Int J Infect Dis 2010;14S1: Abstract number.
Please note that the official publication of the International Journal of Infectious Diseases 2010, Volume 14, Supplement 1
is available electronically on http://www.sciencedirect.com
Final Abstract Number: 28.043
Session: Influenza
Date: Wednesday, March 10, 2010
Time: 12:30-13:30
Room: Poster & Exhibition Area/Ground Level
Type: Poster Presentation
Homologous and heterologous immune responses to naturally-acquired influenza virus infection
L. Lau, V. J. Fang, K. H. Chan, E. Ma, G. M. Leung, M. Peiris, B. J. Cowling
The University of Hong Kong, Hong Kong, China
Background: Humoral antibody responses, typically measured by hemagglutination inhibition
(HAI) and viral neutralization, are common measures of immunity to influenza virus infection. An
antibody titer of greater than 1:40 is thought to correlate to around 50% protection to infection,
while a 4-fold rise in antibody titers between two times is evidence of infection during that interval.
Infection from a specific strain of influenza can elicit an antibody response against other strains of
influenza, which may correlate with protection.
Methods: Subjects presenting with influenza like illness (ILI) were recruited from general
outpatient clinics throughout Hong Kong. Those subjects who tested positive for influenza by a
rapid diagnostic test, and their household members, were followed up with home visits to
determine secondary infections in the household. Serum samples taken at baseline and
convalescence were tested for HAI specific to pandemic H1N1, seasonal H1N1 and seasonal
H3N2. Nasal and throat swabs (NTS) were collected from each individual and tested by RT-PCR
to confirm active infections.
Results: There were 18 pandemic H1N1, 5 seasonal H1N1, and 24 seasonal H3N2 virus positive
cases confirmed by RT-PCR. Among those positive for influenza virus, 39 were adults (16
years) and 8 were children. The proportion of individuals with a homologous antibody titer of
1:40 rose from 17%, 20% and 4% to 78%, 100% and 38% after infection with the pandemic
H1N1, seasonal H1N1 and seasonal H3N2 viruses respectively. Heterologous responses were
strongest between pandemic H1N1 and seasonal H1N1. In individuals with RT-PCR confirmed
seasonal H1N1 infection, the proportion with an antibody titer 1:40 for pandemic H1N1
increased from 20% to 40%. In the opposite direction, the proportion of those confirmed with
pandemic H1N1 with an antibody titer 1:40 for seasonal H1N1 increased from 33% to 44%. Low
levels of cross-strain HAI titer rise to 1:40 were detected in other variations.
Conclusion: We identified low baseline immunity prior to naturally-acquired infection with
seasonal or pandemic strains of influenza in the 2009 summer season in Hong Kong. This
suggests that a proportion of those infected with either seasonal H1N1 or H3N2 viruses could
gain protection against the novel pandemic H1N1 strain and vice-versa.