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 United States, the U.S. Centers for Disease Control and Prevention said on Wednesday.

The U.S. health agency issued an advisory to doctors noting that flu virus samples the agency took from Oct. 1 through Nov. 22, showed that just under half were a good match for the current influenza A (H3N2) component contained in flu shots for the 2014-2015 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 U.S. states.

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

Flu seasons dominated by influenza A (H3N2) strains tend to have higher overall hospitalization 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.

The CDC is stressing that doctors should be prepared to use antiviral medications when needed. These include Roche's Tamiflu and GlaxoSmithKline's Relenza.

However, a recent study from the Cochrane Review, a respected research network, suggests that while these medicines can shorten flu symptoms by around half a day, there is no good evidence behind claims they cut hospital admissions or lessen complications of the disease.

The CDC in its advisory said the benefit of these drugs is greatest when treatment is started early in the course of the infection.