Does previous exposure to influenza H1N1 viruses, either by infection or vaccination, provide any protection against infection with the new H1N1 influenza virus strains? The answer to this question might provide insight as to why over 60% of confirmed cases of influenza caused by the swine-like H1N1 viruses in the US are in 5- to 24-year-olds, as reported at a CDC press conference.
To answer this question, CDC has analyzed serum specimens that were collected during previous vaccine studies. These sera were collected from children and adults before and after they received influenza vaccine in the 2005-06, 2006-07, 2007-08, or 2008-09 influenza seasons. Virus neutralization and hemagglutination-inhibition assays were done to determine whether these sera contain antibodies that cross-react with the new H1N1 strain. The authors of the study used the A/California/04/2009 as a representative of the new H1N1 virus isolates.
The results show that previous immunization of children (age 6 months to 9 years, total of 79 specimens) with either seasonal trivalent inactivated vaccine or infectious, attenuated influenza vaccine of the previous four years did not induce cross-reactive antibody to the new influenza A H1N1 strain. Previous immunization did induce a low cross-reactive antibody response to A/California/04/2009 in adults. Among 18-64 year olds, there was a twofold increase in cross reactivity antibody to the virus, compared with a 12-19 fold increase in antibody titers against the seasonal strains. There was no increase in cross reactive antibodies in adults over 60 years of age. These data indicate that immunization with seasonal influenza vaccines containing previous H1N1 strains (years 2005-2009) is not likely to confer protection against infection with the new H1N1 strains.
An important question is whether the sera obtained before administration of vaccine contain cross-reactive antibody titers against A/California/04/2009. Such analyses would indicate whether natural infection with H1N1 strains confers some protection agains the new isolates. There were no pre-vaccination cross-reactive antibodies to A/California/04/2009 in sera of any of the 79 children of ages 6 months to 9 years. However, 6% of adults 18-40 years old, 9% of adults 18-64 years old, and 33% of adults over 60 years of age had pre-vaccination neutralizing antibody titers to A/California/04/2009 greater than or equal to 160. These antibodies were likely acquired by infection with an H1N1 virus that is antigenically more similar to the A/California/04/2009 than other seasonal H1N1 strains. Whether such antibodies would confer protection against infection is unknown, but they could reduce the severity of disease symptoms.
I suspect that not all readers of virology blog are familiar with the microneutralization and hemagglutination-inhibition assays used in this study. In a separate post, I will explain how the assays work, and the significance of the test results.
J Katz, PhD, K Hancock, PhD, V Veguilla, MPH, W Zhong, PhD, XH Lu, MD, H Sun, MD, E Butler, MPH, L Dong, MD, PhD, F Liu, MD, PhD, ZN Li, MD, PhD, J DeVos, MPH, P Gargiullo, PhD, N Cox, PhD (2009). Serum Cross-Reactive Antibody Response to a Novel Influenza A (H1N1) Virus After Vaccination with Seasonal Influenza Vaccine Morbid. Mortal. Weekly Rep., 58 (19), 521-524