Flu vaccine is 36% effective this season and has been most effective for young children, according to interim estimates the Centers for Disease Control and Prevention (CDC) released today.
“These early VE (vaccine effectiveness) estimates underscore the need for ongoing influenza prevention and treatment measures,” CDC experts said in a new Morbidity and Mortality Weekly Report.
Effectiveness estimates represent the reduction in a person’s risk of getting a flu virus that needs outpatient medical attention. The CDC’s 36% effectiveness estimate is based on data from 4,562 children and adults. The vaccine effectiveness was greatest for children 6 months through 8 years (59%) and adults 18-49 (33%), the only two groups for whom protection was statistically significant. It was lowest for adolescents ages 9-17 (5%).
Vaccine effectiveness for influenza A (H3N2) viruses, which have been predominant this season, are estimated to be 25%, down from 32% last season. For weeks, many have been bracing for low rates following reports of 17% effectiveness against H3N2 in Canada and 10% in Australia.
Vaccine effectiveness is higher for other strains of the virus — 67% against influenza A (H1N1)pdm09 and 42% against influenza B, according to the report.
CDC experts say a person’s age, infection and vaccination history can impact immune responses to the vaccine, and they called for additional research into differences in egg-based vs. non-egg-based vaccines.
“… many efforts are under way to improve selection and development of candidate vaccine viruses that are optimal for vaccine production and provide protection against a majority of circulating viruses,” they wrote.
This flu season has been severe, comprising 7.7% of outpatient clinic and emergency department visits, which matches the peak of the 2009 H1N1 pandemic, according to the CDC’s latest data. The hospitalization rate for flu-related illness has been nearly 60 per 100,000 population, the highest since tracking started in 2010. Sixty-three children have died.
The CDC continues to encourage everyone 6 months and older to be vaccinated as there may be several weeks left in the season.
“Even with current vaccine effectiveness estimates, vaccination will still prevent influenza illness, including thousands of hospitalizations and deaths,” according to the report.
People who are very old, very young, pregnant women and people with chronic conditions are at highest risk of complications. Signs of severe flu include persistent high fever, difficulty breathing, rapid heartbeat, significant fatigue and feeling better followed by feeling much worse. Clinicians who suspect flu in these patients should begin antiviral treatment immediately.