The New England Journal of Medicine late yesterday published reports showing that inactivated 2009 H1N1 influenza vaccines appear to produce immunity in adults after a single dose. Preliminary analysis of early data from NIH trials appears to align with other findings that a single dose of H1N1 vaccine induces a strong and prompt immune response in most healthy adults. This is good news because, if confirmed and FDA-approved, we will be able to protect a much larger proportion of the population more quickly and conviently, as adult patients may not need a second dose. Similar findings are expected in young adults age 18 and older.
Results in children are not yet available; though vaccine-induced immunity after a single dose of seasonal flu vaccine is limited for children under the age of 9, making it more likely that children may need two doses of H1N1 vaccine. Data from adults was unexpectedly positive; we cannot predict with confidence what the pediatric data will show about the necessity of one vs. two doses.
Basically, this past year we have had two flu seasons – first the usual 2008-9 season, then the 2009 novel H1N1 influenza season, which started in April and has, essentially, continued until now. It appears likely that we will have two vaccines to address this – one, the seasonal vaccine, which is already available, and the second, the 2009 H1N1 vaccine.