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Flu vaccine 47% effective in US, early estimates show
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The influenza vaccine has been 47% effective overall this season in the United States, including 46% effective against the predominant circulating virus, influenza A(H1N1), but has provided “limited to no” protection in older adults, according to interim estimates published today in MMWR.
The vaccine’s estimated overall effectiveness compares well with past seasons, and is higher than three of the past four. Elsewhere, researchers reported recently that the vaccine has been 68% effective in Canada, including 72% effective against H1N1. Interim estimates from Hong Kong showed the vaccine has been 90% effective overall in children, including 92% effective against H1N1.
According to Joshua D. Doyle, MD, PhD, Epidemic Intelligence Service officer in the CDC’s Influenza Division, and colleagues, the vaccine has been 61% effective overall this season in U.S. children aged 6 months to 17 years (95% CI, 44%-73%) and 62% effective against H1N1 (95% CI, 40%-75%).
However, it has been just 24% effective overall in adults aged 50 years or older (95% CI, –15% to 51%), including only 8% effective against H1N1 (95% CI, –59% to 46%), the estimates show.
“The numbers [of infections included in the analysis] are still small but the idea that we are seeing limited to no evidence of protection for those over 50 is certainly something we want to look at very carefully,” Michael T. Osterholm, PhD, MPH, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, told Infectious Disease News.
Osterholm explained that when an age group — specifically adults aged 50 years or older — has high levels of vaccination, providing precise estimates on vaccine effectiveness is more difficult.
As the influenza season progresses, the number of cases will increase, allowing for better measurement of effectiveness, he said.
“We expect to see, over the course of the season, larger numbers, and then we’ll get a better read out of what’s going on,” Osterholm noted. “Right now, it’s a challenge to see that we have limited evidence of protection in this age group.”
The interim U.S. estimates are based on data from 3,254 children and adults enrolled in the U.S. Influenza Vaccine Effectiveness Network between Nov. 23 and Feb. 2 from five study sites in Michigan, Pennsylvania, Texas, Washington and Wisconsin.
Vaccination coverage has varied by study site, sex, age group, race/ethnicity and interval from illness onset to enrollment, and ranged from 46% to 61% across the five study sites. Among participants with influenza, 43% received the 2018-2019 seasonal influenza vaccine compared with 57% among those who did not have influenza.
Another MMWR authored by CDC epidemiologist Lenee Blanton, MPH, and colleagues detailed influenza activity for the U.S. between Sep. 30 and Feb. 2 and reported low severity for the 2018-2019 season. Despite this, preliminary cumulative in-season prevalence estimates found that between 155,000 and 186,000 hospitalizations and 9,600 to 15,900 deaths have been caused by influenza this season. The researchers noted that parts of the U.S. may have elevated influenza activity for several more weeks.
“It’s not too late to get your flu shot,” Osterholm said. “There still is growing activity, right now. This week’s activity was higher than last week’s and in the next week, if people go and get their flu shot, it can prevent substantial morbidity and mortality.”
Last season, influenza caused 48.8 million illnesses, 22.7 million medical visits, 959,000 hospitalizations and 79,400 deaths — including 183 confirmed pediatric deaths — according to the CDC. As previously reported, last season’s influenza vaccine was 38% effective overall and 22% effective against influenza A(H3N2), which was the predominant circulating virus. Vaccination prevented an estimated 7.1 million illnesses, 3.7 million medical visits, 109,000 hospitalizations and 8,000 deaths last season.
Osterholm said more work has been done in the last 2 to 3 years to develop universal, game-changing influenza vaccines than was completed in the previous 20 years, but a finished product will still take a while.
“This is an exciting time to be in influenza vaccine research work,” he said. “In the meantime, the message we should emphasize is to continue to get vaccinated; even some protection is better than nothing,” he said. – by Marley Ghizzone
References:
Blanton L, et al. MMWR Morb Mortal Wkly Rep. 2019;doi:10.15585/mmwr.mm6806a1.
Chiu SS, et al. Euro Surveill. 2019;doi:10.2807/1560-7917.ES.2019.24.5.1900056.
Doyle JD, et al. MMWR Morb Mortal Wkly Rep. 2019;doi:10.15585/mmwr.mm6806a2.
Rolfes MA, et al. Clin Infect Dis. 2019;doi:10.1093/cid/ciz075.
Skowronski DM, et al. Euro Surveill. 2019;doi:10.2807/1560-7917.ES.2019.24.4.1900055.
Disclosures: Blanton, Doyle and Osterholm report no relevant financial disclosures. Please see the study for all other authors’ relevant financial disclosures.