What makes for an effective flu vaccine? Researchers from the National Institute of Allergy and Infectious Diseases (NIAID) tested the conventional wisdom about what makes flu vaccines work and found some “interesting clues.”
Researchers have long thought that higher levels of antibodies against hemagglutinin (HA) are key. The HA protein helps the virus enter the human cell and initiate infection. But according to the NIAID research, higher levels of antibody against neuraminidase (NA) better predict who will have to weather the effects of mild-to-moderate flu. Neuraminidase allows newly formed flu viruses to exit the host cell and cause further replication in the body.
The researchers divided 65 healthy volunteers into 2 groups, based on their levels of anti-HA antibodies. The participants were given an intranasal dose of 2009 H1N1 influenza virus, monitored for 9 days, and followed for 8 weeks.
Not surprisingly, those who had high levels of anti-HA antibodies had a significantly lower incidence of mild-to-moderate flu, and the flu’s duration was somewhat shorter. But those participants were just as likely to have some flu symptoms as those with low levels of HA antibodies. That suggests, the researchers say, that high HA antibody levels may limit viral shedding—and the spread of virus—but may not prevent the flu.
High NA antibody levels, however, had a “more robust” effect. Participants with high NA antibody levels had less severe disease, shorter duration of viral shedding and symptoms, and fewer and less severe symptoms.
HA and NA antibody levels together may be a better predictor of whether someone develops mild-to-moderate flu than either alone, the researchers conclude, but NA antibodies are the best way to predict disease severity.