Less Effective Vaccine May Mean More Severe Influenza Season

December 04, 2014

The 2014 to 2015 influenza season could be more severe than most — with more hospitalizations and deaths — because the current vaccine looks to be less effective than expected against the dominant influenza strain in circulation, according to the Centers for Disease Control and Prevention (CDC).

Influenza A(H3N2) viruses account for about 90% of infections, said CDC Director Tom Frieden, MD, MPH, in a news conference today. The trivalent seasonal influenza vaccine contains this strain, which matches well to half of the A(H3N2) viruses identified so far. The other half, however, are antigenically different from the version of the A(H3N2) virus chosen for the vaccine.

"They're different enough that protection may be lower than what we usually see," said Dr Frieden. "It is possible we could have a flu season that is more severe than most."

Reiterating the findings of a CDC health advisory issued yesterday, Dr Frieden said the slight change, or "drift," in the A(H3N2) virus used in the vaccine was detected in March, which was after the 2014 to 2015 vaccine had been formulated. "It was already too late to include [the drifted virus] in the vaccine," he said.

A less effective vaccine compounds what could be a tougher influenza season anyway. Historically, influenza seasons predominated by the A(H3N2) virus tend to have twice as many hospitalizations and deaths as seasons when other viruses are in the forefront, said Dr Frieden.

Only time will tell, he said, whether current trends in virus circulation hold true for the rest of the influenza season. Influenza B viruses now represent almost 9% of the viruses tested by the CDC and match up well with the B strain in the vaccine. The other 91% are influenza A viruses, and nearly all of them are A(H3N2), with a smattering of the once-pandemic A(H1N1) virus, which also is included in the vaccine.

Dr Frieden stressed that the current vaccine, although impaired, remains the single best safeguard against influenza and "may have some effectiveness" against the drifted A(H3N2) virus. Yesterday's CDC advisory made that same point, saying that the vaccine "may reduce the likelihood of severe outcomes such as hospitalization and death" for people infected with the drifted virus.

The vaccine in the 2013 to 2014 season reduced the risk of catching the influenza by 50% to 55%, said Joseph Bresee, MD, an epidemiologist in the CDC's National Center for Immunization and Respiratory Diseases, at the news conference. That level of effectiveness matched historical norms, said Dr Bresee.

Antivirals Underprescribed

Given the possibility of a more severe influenza season, Dr Frieden today reminded clinicians to treat suspected cases with the antivirals oseltamivir and zanamivir, especially when patients are at high risk for complications because of a chronic condition such as asthma or diabetes. The drugs should be prescribed as quickly as possible, and ideally within 48 hours, when someone has influenza-like symptoms.

"Treatment with antiviral drugs can make your illness milder and shorter," he said. "It can reduce the likelihood that you'll end up a hospital or an intensive care unit...and reduce the risk of dying from influenza."

Clinicians should not wait for a laboratory test to confirm the diagnosis for influenza before writing an antiviral prescription, said Dr Frieden. The mistaken belief that such a delay is necessary, he said, helps explain why antivirals are widely underprescribed.

At this early stage of the current season, influenza activity generally is low across the country but is picking up steam in a few Southern states, notably Louisiana and Alabama, according to the CDC. As of November 22, the proportion of deaths attributed to influenza and pneumonia nationwide was below what the CDC calls the epidemic threshold.


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