A vaccine used to immunize 45 million people against an influenza virus in 1976 appears to have protected some of those same individuals from a genetically similar virus that caused a pandemic in 2009, according to a study by scientists at the U.S. Food and Drug Administration (FDA).
The 1976 virus caused an outbreak of “swine flu” at a military base in New Jersey, prompting development of the vaccine due to fear that the outbreak might lead to a pandemic. While the 1976 outbreak was limited to the base, the recent 2009 H1N1 pandemic influenza virus spread rapidly around the world. However, in the U.S., there were fewer deaths among older adults from this new virus, especially among those 65 years of age and older who in general are more susceptible to influenza than younger adults.
The results of this study suggest that the 1976 swine flu vaccine might have triggered production of antibodies in recipients of that vaccine that protected them as older adults from the 2009 pandemic influenza virus.
The FDA scientists discovered that one type of antibody triggered in recipients of the 1976 vaccine was able to cross-react with the 2009 influenza virus, that is, the antibodies could recognize a virus protein called HA on the 2009 virus; a second type of antibody triggered by the 1976 vaccine cross-reacted with another protein on the 2009 virus, called NA.
Scientists generally agree that an influenza vaccine is most effective against a particular influenza virus if it triggers production of antibodies that specifically recognize and attack the HA protein of that virus. Scientists also agree that antibodies targeting the NA protein of influenza viruses are not as critical as HA antibodies in fighting against influenza. However, HA proteins frequently mutate, making it harder or impossible for antibodies against the original proteins to recognize the mutated ones. This is why seasonal influenza vaccines are updated each year. Unlike HA, NA is relatively stable and does not readily mutate.
In order to study the antibodies from the 1976 vaccinations, the scientists used samples of serum that had been collected from recipients of the vaccine and stored at the Center for Biologics Evaluation and Research (CBER). The study shows that test mice given sufficient antibodies cross-reacting with the HA protein of the 2009 virus survived the lethality of the recent 2009 H1N1 pandemic virus.
The scientists also found that antibodies from the 1976 vaccine recipients that cross-reacted with the NA protein of the 2009 virus were able to protect mice in the absence of matched HA antibodies. This suggests that vaccination induced NA antibodies might provide additional protective benefits when seasonal vaccines are not optimally matched with the HA of circulating viruses. Therefore, it might be worthwhile to pursue further investigations on the contribution of NA antibodies to the effectiveness of influenza vaccines.
“Revisiting the 1976 “Swine Flu” Vaccine Clinical Trials: Cross-reactive Hemagglutinin and Neuraminidase Antibodies and Their Role in Protection Against the 2009 H1N1 Pandemic Virus in Mice”
Clinical Infectious Diseases 2011:53: 1179-1187
Hang Xie, Xing Li, Jin Gao, Zhengshi Lin, Xianghong Jing, Ewan Plant, Olga Zoueva, Maryna C. Eichelberger, and Zhiping Ye
Laboratory of Respiratory Viral Diseases, Division of Viral Products, Office of Vaccines Research and Review, Center for Biologics Evaluation and Research, U.S. Food and Drug Administration, Bethesda, Maryland