Vaccine Efficacy Questionable Against Predominant Flu Virus

December 12th, 2017

By Gary Evans, Medical Writer

Influenza activity is higher than normal thus far this season, and most cases are being caused by a virus that may be ill matched to the vaccine, the Centers for Disease Control and Prevention reports.

“Influenza A (H3N2) viruses were most commonly reported during week 48 – the week ending December 2, 2017 – and have been the predominant virus so far this season,” the CDC reported. "Of the 347 influenza A viruses that were subtyped, 314 (90.5%) were H3N2 viruses and 33 (9.5%) were (H1N1) pdm09 viruses.

Based on the flu season in Australia, which used the same vaccine now recommended in the United States, immunization may only afford scant protection against H3N2, says Anthony Fauci, MD, director of the NIH National Institute of Allergy and Infectious Diseases (NIAID).

“Since we are using the same vaccine that was used in Australia and it looks like the virus now is similar to the virus that was circulating in Australia, it is likely – not definite, but likely – that we will have a rough season with influenza,” he says. “Whether [the vaccine] is going to be 10% effective – or 15%, 20% or 5% – it is very difficult to put an exact number on it. But the one thing that we do know is that the vaccine that was grown in eggs is a bit of a mismatch with the circulating virus.”

That said, flu immunization is still strongly recommended to protect against other circulating strains and gain as much immunity as possible to the H3N2 strain. As the old adage notes, even a partially effective flu shot can keep you out of the hospital or the morgue.

For more on this story, see the January 2018 issue of Hospital Infection Control & Prevention.