CDC: Flu vaccine is only 23 percent effective this season

e virus only 23 percent of the time, a low level of protection compared to years when the flu shot better matched the circulating strains, the Centers for Disease Control and Prevention said Thursday.

The problem is that more than two-thirds of the viruses are of a different strain, or “drifted,” from the type used to devise the vaccine, according to the government.

“Overall, the estimate suggests that the 2014–15 influenza vaccine has low effectiveness against circulating [strains],” the agency said in its Morbidity and Mortality Weekly Report.

The government said it could estimate the vaccine’s effectiveness because of early and widespread circulation of influenza. Nearly all the states saw widespread flu activity as of Jan. 3.

The researchers based their conclusions on a study of more than 2,300 people in five states who had respiratory illnesses requiring hospitalization since November. People who had been vaccinated had a 23 percent lower chance of needing to see a doctor for the flu.

The report said the vaccine still provides protection against some strains, or some that may crop up. A good match would see the vaccine provide about 50 percent to 60 percent fewer doctor visits for influenza.

Every year the federal government has to reformulate the flu vaccine based on the best guesses of the CDC and other experts on which flu strains will be most widespread. But the time required to make enough flu shots and nasal sprays requires that those guesses be made in February, months before the flu season begins in late fall.

This year’s formulation was not aimed at the H3N2 strain of the virus, and it wound up causing a majority of this winter’s cases.

Nonetheless, the CDC said the vaccine’s relatively poor match underscores the need for effective prevention and treatment of flu symptoms, particularly among young children, adults older than 65 or people with medical conditions that put them at extra risk. They also noted that H3N2 is a particularly virulent strain in terms of causing hospitalizations and deaths, especially among older patients.

“Physicians should not wait for confirmatory influenza laboratory testing, and the decision to use an antiviral medication should not be influenced by patient vaccination status,” the report said.