Mismatched Flu Strains? No Problem

Flu vaccine effective even when mismatched for circulating flu strains that season

/ Author:  / Reviewed by: Joseph V. Madia, MD Beth Bolt, RPh

(RxWiki News) Each year a new flu vaccine is created to try to match up with the strains of flu that are going around. What happens if the flu vaccine isn't a perfect match for the strain of virus it's fighting?

A recent study found that the flu vaccine can still offer protection against flu infections even when the strains don't match.

Vaccines are made with pieces of the flu virus or inactivated viruses that don't cause disease. The body builds up its defenses against the virus strains in the vaccine so it's ready when faced with the real virus.

Even if the flu strains making people sick that season are different from the ones included in that year's flu vaccine, the shot can still be effective.

This study showed that the effectiveness was lower when the strains didn't match, but was still above 50 percent in most situations.

"Ask your doctor about the flu vaccine."

The study, led by Andrea C. Tricco, PhD, from the Li Ka Shing Knowledge Institute at St. Michael’s Hospital in Toronto, Canada, looked at the effectiveness of flu vaccines over multiple years and in different studies.

The researchers looked for all randomized clinical trials involving flu vaccines in three major research databases.

They specifically selected all trials in which individuals had gotten the flu vaccine and contracted influenza that had been confirmed by lab work.

Some of the individuals caught the same strain of the flu they had been vaccinated against, and others caught a different strain not included in the vaccine they had received.

The researchers identified 34 randomized, controlled trials that involved 94,821 total participants across 47 different flu seasons.

The researchers calculated the flu vaccine's effectiveness even when the flu strains that were circulating most during a particular flu season were not the ones covered by that season's vaccine.

They found that among children aged 6 months to 3 years old, the flu vaccine was about 83 percent effective in preventing children from getting the flu if the strains in the vaccine matched those circulating. This finding was based on an analysis of the combined results of seven studies.

Even if the vaccine strains did not match the strains circulating, however, the vaccine was still 54 percent effective in preventing infections in this age group, based on analysis of six studies.

It did appear to matter which circulating strain was the mismatched one, though.

Two main types of influenza viruses cause the flu each season: influenza A and influenza B.

Both influenza A and influenza B have different strains. These are different varieties of the virus similar to the way there are different breeds of dogs.

Scientists never know exactly which strains will be making people sick each year. They make predictions based on early testing and past trends. Then they choose certain influenza A and influenza B strains to make the vaccine. It's always possible that different A and B strains will be floating around than those in the vaccine.

If influenza A strains were circulating but did not match the strains used in the vaccine, then the flu vaccine was 75 percent effective in preventing the flu for children 6 months to 3 years old.

However, if the influenza B strains that were circulating did not match the ones used in the vaccine, then the vaccine was only 42 percent effective in preventing flu in children in that age group.

The flu vaccines also offered protection to adults even when the circulating strains mismatched the vaccines.

Based on an analysis of eight studies, the flu vaccine was 65 percent effective in preventing flu in adults when the strains matched.

When they didn't match, based on data from nine studies, the vaccine was still 52 percent effective in preventing flu infections in adults.

Again, mismatched strains of influenza A were more effective (64 percent effective, based on five studies) than mismatched strains of influenza B were (52 percent effective, based on eight studies).

The authors concluded that flu vaccines "...can provide cross protection against non-matching circulating strains."

Even when the strains used to make a vaccine do not match the strains circulating in the general population that flu season, the shot still helps prevent infections.

The study was published June 25 in the journal BMC Medicine. The research was funded by GlaxoSmithKline, which also manufactures vaccines.

Five authors have received consulting fees from GlaxoSmithKline, and two authors are paid employees of the company and own company stock. One author has received a previous grant from GlaxoSmithKline, and two others have received travel funds to attend meetings.

Review Date: 
June 26, 2013
Last Updated:
September 6, 2013