Available vaccine possibly ineffective as influenza virus may have drifted US agency says current shots may not be a good match for flu strain

A sampling of flu cases so far this season suggests the current flu vaccine may not be a good match for the most common seasonal flu strain currently circulating in the US, the US Centres for Disease Control and Prevention (CDC) said.

Flu virus samples the agency took from October 1 through November 22 showed that just under half were a good match for the current influenza A (H3N2) component contained in flu shots for the 2014-15 season, suggesting the virus has drifted.

According to the CDC, flu activity has been low but is increasing and influenza A (H3N2) viruses appear to be the predominant strain, with cases having been detected in almost all US states.

Flu shots may still offer some protection

In past seasons when the influenza A (H3N2) strain had mutated from the vaccine strain, flu shots were less effective, the CDC said.

Flu seasons dominated by influenza A (H3N2) strains tend to have higher overall hospitalisation rates and more flu-related deaths, especially among older people and very young children compared with flu seasons dominated by the influenza A (H1N1) virus or influenza B viruses.

The CDC said flu shots may still offer some protection against drifted viruses, which could help reduce the risk of the most severe complications from the flu, such as hospitalization and death.

The current seasonal flu shots will still protect against flu strains that have not mutated, such as the influenza A (H1N1) virus and the B viruses contained in the vaccine.