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Good News and Bad News About 2009 H1N1 Influenza ("Swine Flu")

Only a minority of stored serum samples in the U.S. contained antibodies against H1N1, but a new vaccine easily elicited them.

As observers wait for the 2009 pandemic influenza A (H1N1) virus to strike during the upcoming flu season, a few data points finally are available to temper the dire predictions.

Investigators from the CDC tested stored sera from recent flu vaccine trials to determine the prevalence of natural immunity to 2009 pandemic H1N1 and the effect of previous vaccination. Serum samples from 100 infants and children who received flu vaccines during the past 4 years were evaluated: Preexisting immunity to pandemic H1N1 was uniformly negligible, and vaccination with standard seasonal vaccines did not boost titers. Among 393 serum samples from adults, 6% to 33% exhibited substantial pandemic H1N1 antibody titers before vaccination, and 7% to 43% contained them after vaccination. A survey of serum samples from blood donors showed that 11 older donors (born between 1912–1920) all had high titers of pandemic H1N1 antibodies; 39 of 115 donors (34%) who were born before 1950 had high titers, and 45 of 83 (54%) who had received swine flu vaccine in 1976 had high titers.

On the vaccine front, British investigators found that antibody titers among 25 subjects who received one dose of an inactivated H1N1 vaccine were similar to those among 75 subjects who received two doses; protective titers were achieved uniformly by day 21. In an Australian study of a slightly different vaccine formulation, researchers found that high H1N1 antibody titers were present in 30% of healthy adults at baseline and in 95% at 21 days after vaccination. Somewhat lower titers were found among older adults (age range, 50–64) than among younger adults. Neither vaccine study revealed evidence of serious adverse events; in both, local and systemic reactions (such as headache and myalgias) were reported commonly.

Comment: These data support clinical observations: Children and young adults have little natural protection to pandemic H1N1, even after routine seasonal flu vaccination. Fortunately, however, a single dose of vaccine probably will be sufficient to achieve seroconversion and, we hope, clinical protection from the worst of the illness.

— Abigail Zuger, MD

Published in Journal Watch General Medicine September 17, 2009

Citation(s):

Hancock K et al. Cross-reactive antibody responses to the 2009 pandemic H1N1 influenza virus. N Engl J Med 2009 Sep 10; [e-pub ahead of print]. (http://dx.doi.org/10.1056/NEJMoa0906453)

Clark TW et al. Trial of influenza A (H1N1) 2009 monovalent MF59-adjuvanted vaccine — Preliminary report. N Engl J Med 2009 Sep 10; [e-pub ahead of print]. (http://dx.doi.org/10.1056/NEJMoa0907650)

Greenberg ME et al. Response after one dose of a monovalent influenza A (H1N1) 2009 vaccine — Preliminary report. N Engl J Med 2009 Sep 10; [e-pub ahead of print]. (http://dx.doi.org/10.1056/NEJMoa0907413)

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