CDC Reports That Flu Vaccine Has Been 36% Effective This Season

As widespread flu activity in the United States has continued in all states but Hawaii and Oregon, the Centers for Disease Control and Prevention (CDC) reported 22 pediatric flu deaths during week 6, ending February 16, 2018—the most reported in any 1 week this season.

Of the newly-reported pediatric flu deaths, 17 were from influenza A viruses and 5 were from influenza B viruses, bringing the total this season to 84 pediatric deaths. The percentage of outpatient visits reported for influenza-like illness remained elevated throughout all regions of the country but declined slightly from 7.7% the previous week to 7.5% in week 6. The rate of respiratory specimens positive for influenza B viruses has continued to rise, making up 36.4% of more than 17,000 laboratory-tested specimens coming up positive for flu in all 50 states, Puerto Rico, and the District of Columbia.

In its Morbidity and Mortality Weekly Report for February 16, the CDC released its interim estimates of seasonal vaccine effectiveness for the 2017-2018 flu season. Using data collected from 4,562 children and adults enrolled in the US Influenza Vaccine Effectiveness Network from November 2, 2017, to February 3, 2018, the CDC has found that, overall, the vaccine has been 36% effective against influenza A and influenza B viruses. For influenza A (H3N2), the dominant strain this flu season, the vaccine has been only 25% effective at preventing illness. However, the vaccine has performed better against other circulating viruses; it has been 67% percent effective against influenza A(H1N1)pdm09 viruses and 42% against all influenza B viruses.

“These early VE estimates underscore the need for ongoing influenza prevention and treatment measures,” authors of the report write. “CDC continues to recommend influenza vaccination because the vaccine can still prevent some infections with currently circulating influenza viruses, which are expected to continue circulating for several weeks. Even with current vaccine effectiveness estimates, vaccination will still prevent influenza illness, including thousands of hospitalizations and deaths.”

According to the CDC, as of November 2017, approximately 38% of infants, children, and adults in the United States had received the flu vaccine for the current flu season. Despite continued efforts by health officials to get individuals to receive the flu shot, vaccination coverage remains low for the vaccine, in part due to its rate of effectiveness. Now, a new study published in the journal Clinical Infectious Diseases suggests that immune history may play a role in the effectiveness of the flu vaccine. The flu virus is a moving target, changing from season to season, and requiring new vaccines capable of targeting specific strains. This, along with the egg-based vaccine production method that makes the virus prone to mutation, has made it hard for researchers to develop a highly-effective flu vaccine.

Once an individual has been exposed to the flu for the first time, through infection or vaccination, the new study suggests that his/her immune system essentially wants to keep making the same antibodies produced the first time, and those old antibodies may be ineffective against new viruses. It’s a phenomenon the authors call “original antigenic sin,” and it may explain why some vaccinated individuals still get sick even when a seasonal vaccine is a good match to circulating viruses.

"We need to do more basic research on how to induce responses to the right sites on the virus, and this will require us to understand original antigenic sin better," said lead author Sarah Cobey, PhD, in a recent press release. "We also need to understand why the vaccine appears to be bad at eliciting responses in some people some of the time. Is there really no response, or are we just not looking in the right places?"