CDC Releases New Estimates on 2016-2017 Flu Vaccine's Effectiveness

With several more weeks left to go in the 2016-2017 flu season in the United States, the Centers for Disease Control and Prevention (CDC) has released early data showing that this year’s flu shot has effectively reduced the risk of being infected with the virus by nearly half for those vaccinated.

The new estimates on the flu vaccine’s effectiveness (VE) for this season were recently published in the latest issue of the CDC’s Morbidity and Mortality Weekly Report. The authors of the report examined data collected on 3,144 children and adults at five study sites enrolled in the US Influenza Vaccine Effectiveness Network from November 29, 2016 to February 4, 2017. The study participants were individuals aged 6 months or older who had sought medical care for acute respiratory illness, and the percentage of those vaccinated with the seasonal flu shot ranged from 46% to 61% at the five study sites. Of the 3,144 individuals in the study group, 744 (24%) had laboratory-confirmed cases of influenza; 88% of these were cases of infection with influenza A.

The researchers found that this season’s flu vaccine had a 48% rate of effectiveness, after adjusting for factors such as study site, age group, sex, ethnicity, self-rated general health, and number of days from illness onset to enrollment. During a flu season dominated by the influenza A H3N2 strain—known to cause more severe illness for those infected—the CDC report notes that the vaccine has had a 43% effectiveness rate so far against the predominant influenza A H3N2 virus, and a 73% effectiveness rate against circulating influenza B viruses. These findings are on par with vaccine effectiveness estimates for past seasons’ vaccines such as the 2012-2013 seasonal flu vaccine, which the CDC said was 49% effective and prevented 5.6 million flu illnesses, 2.7 million flu-related medical visits, and an estimated 61,500 flu hospitalizations.

“The early VE estimates also underscore that some vaccinated people may get influenza,” according to a news release on the findings. This statement follows a recent study which indicated that effectiveness may be reduced in those who get the vaccine for consecutive seasons. Health officials also noted that getting vaccinated now may still prevent illness, as there are several weeks left in the current flu season. “In the past couple of weeks, a small increase in the proportion of influenza B viruses has been detected. During many seasons, there are later waves of influenza B virus activity. In the past, significant influenza activity has been observed as late as May. Vaccination efforts should continue as long as influenza viruses are circulating.”

Flu season is indeed still in full swing for much of the country, including in Idaho, where local health officials say they are experiencing one of the most severe flu seasons they have seen in recent years. “We are aware of 47 influenza-related deaths in Idaho so far this season, which includes 36 verified flu deaths and 11 current reports that are in the process of verification,” said state influenza surveillance coordinator Leslie Tengelsen, DVM, PhD, in a recent press release. “This is one of the most severe flu seasons in the state since 2000.” While the flu vaccine is not 100% effective at preventing illness, Dr. Tengelsen notes that getting the shot is still the best way to avoid contracting the virus this season.