For Past Seasonal Flu, Shot Vaccine More Effective Than Nasal Spray

If given a choice between the injected vaccine and the nasal-spray vaccine for either seasonal influenza or pandemic influenza, you may want to opt for the shot. But only if you're a grown-up.

That's because...

The injected vaccine for seasonal influenza was more effective at inducing immunity than the nasal-spray version in adults, according to a study published yesterday in the NEJM. The results can possibly be applied to vaccine options for pandemic H1N1. (But then again, maybe not. Read on.)

In a randomized, double-blind, placebo-controlled study of 1952 healthy adults (18-49 years of age) during the 2007-2008 influenza season,* the injected vaccine (which contains inactivated virus) reduced the relative risk of laboratory-confirmed influenza by 50%, when compared with the nasal-spray vaccine (which contains live, attenuated virus). The rates of absolute efficacy against influenza A (predominately the H3N2 virus) were 72% and 29% for the shot and spray vaccines, respectively.

Other data indicate, however, that the spray vaccine is more effective for the prevention of flu than the injection in young, vaccine-naive children (2 doses) and in older children (1 dose).

It is speculated that the nasal-spray vaccine is relatively less effective in adults, because preexisting seasonal flu antibodies thwart the vaccine antigens at their entry point in the nasal passages. But because the pandemic H1N1 virus is new to everybody, the nasal-spray vaccine may work comparably in children and adults.

* According to the CDC, circulating viruses during this season in the United States were influenza A (71%) and influenza B (29%). Early in the season, influenza A H1N1 viruses predominated, followed by an increase in circulating H3N2 viruses. Influenza B viruses were more common during the tail of the season. Circulating H3N2 viruses were most common overall.

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And on a related note...

A high-profile anecdote of confirmed illness with the pandemic H1N1 virus was provided yesterday by CNN's Sanjay Gupta. Gupta's prominent symptoms were similar to those of seasonal flu: a hacking cough, fever, nausea, and body aches. Although Gupta reported, "[T]his was the sickest I have ever been," he recuperated after a couple of days of symptomatic treatment and bed rest. He evidently did not receive antiviral therapy (eg, oseltamivir [Tamiflu]).