T

he reputation of the flu vaccine has taken a bit of a beating in recent years. Official claims about its effectiveness have been downgraded. And some years — like last year — the vaccine hasn’t been well-matched to the influenza viruses making people sick.

Now research points to a single component as the weak link in the vaccine, which is actually designed to protect against three or four types of influenza viruses.

Data being presented at a infectious diseases conference in San Diego this week show that the part of the vaccine targeting a strain known as influenza A H3N2 has cut the risk of infection by an average of only 38 percent in recent years.

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The new research suggests the vaccine offers substantially stronger protection against the H1N1 subtype and influenza B — cutting the risk of infection by 60 percent to 75 percent.

The relative weakness of the vaccine against H3N2 is problematic, and not only because it has the potential to erode public confidence in the vaccine overall. As a family, H3N2 viruses cause more severe illness. When these viruses dominate, flu seasons are typically severe.

“H3N2 is clearly where we have the biggest problem and that’s also of greatest concern because H3N2 is the virus that accounts for most of the hospitalization and mortality in seniors,” said Dr. Edward Belongia, an influenza epidemiologist at Marshfield Clinic Research Foundation in Marshfield, Wis.

Belongia is the lead scientist on the metastudy, which examined data from 60 studies from around the world conducted in the past eight years. Results are being presented at IDWeek, the annual scientific meeting of the Infectious Diseases Society of America, the Society for Healthcare Epidemiology of America, the HIV Medicine Association, and the Pediatric Infectious Diseases Society.

Until about a decade ago, the flu vaccine was generally said to cut one’s risk of catching influenza by between 70 percent and 90 percent. That estimate was based on a few studies that had been conducted years earlier in healthy adults.

But starting in about 2005, flu epidemiologists began using a new type of study design to assess the vaccine’s impact during individual flu seasons. The result: real-time estimates of how well the vaccine was working, broken down by the virus type targeted.

The flu shot is meant to protect people against two influenza A viruses — H1N1 and H3N2 — and one or two types of influenza B viruses. Previous estimates of its effectiveness had lumped all the vaccine components together.

“When you average it, you mask the variability,” says Dr. Danuta Skowronski, a flu epidemiologist at the British Columbia Center for Disease Control in Vancouver and one of the pioneers of the new study design.

These types of studies also allow researchers to estimate how well the various components of the vaccine work in different age groups.

While Belongia’s meta-analysis suggests children have received better protection against the H3N2 strain than seniors — the vaccine has cut children’s risk of getting sick by 46 percent and seniors’ risk only 36 percent — the H1N1 and B vaccines still elicit stronger protection.

“Nobody responds all that well to H3N2 vaccine. The vaccine has some effectiveness and 30 to 40 percent effective is better than nothing, but it’s certainly not as good as we would like to see,” Belongia said.

Influenza specialists at the Centers for Disease Control and Prevention in Atlanta agree.

Dr. Daniel Jernigan, director of the agency’s influenza division, notes that since these studies only date to 2005, it is not known how effective — or not — H3N2 vaccine was before then. Still, he said, it’s clear a better vaccine is needed.

While Belongia’s study found that the H3N2 component of the flu vaccine had cut the risk of infection by 38 percent over time, some years it didn’t fare as well.

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