A systematic review of nearly a decade of influenza studies across the globe shows influenza vaccines provided substantial protection against the H1N1 strain that has circulated since the 2009 pandemic, and against type B viruses. The study also found that vaccination generates lower protection against the H3N2 viruses that have been infecting humans since the 1960s.
The findings are based on a report published in The Lancet Infectious Diseases. The analysis, which included studies published from 2007 through early 2015, was conducted by Edward Belongia, MD and colleagues at Marshfield Clinic Research Foundation (MCRF) and the University of Minnesota.
The research team screened 3,368 studies and selected 56 that met strict criteria for assessing vaccine effectiveness against different flu subtypes. Seasonal flu vaccines are updated each year and contain two subtypes of influenza A virus (H3N2 and H1N1) and one or two strains of influenza B.
The study team found that seasonal vaccines provided 61 percent protection against the H1N1 strain and 54 percent protection against type B. Vaccine protection was only 33 percent for the H3N2 strain. Although vaccine effectiveness can decline in older adults due to aging of the immune system, the review found that protection against H1N1 and type B was maintained even in the oldest age groups.
“It’s reassuring to see that flu vaccines around the world consistently provide moderately high protection against two major flu subtypes,” said Belongia, director, MCRF Center for Clinical Epidemiology & Population Health. “This year we are seeing mostly H1N1 and B infections, and mid-season estimates from the U.S. and Canada show substantial vaccine protection as expected based on our systematic review.”
Influenza seasons are unpredictable with circulation of different virus subtypes. Seasons with mostly H3N2 viruses tend to be more severe, and H3N2 evolves more rapidly to evade the immune system compared to other flu viruses.
“The reasons for lower vaccine protection against H3N2 are unclear, but we suspect continuous virus evolution in nature and egg-based vaccine production may be contributing,” Belongia said. When H3N2 vaccine strains are adapted to grow in eggs, mutations can occur that affect the ability of the immune system to target the virus.
The review included studies that measured vaccine effectiveness by comparing vaccination frequency in patients with lab confirmed flu and in control patients who had a similar illness but tested negative for the flu virus. This “test-negative” approach was first used in 2005 and has been widely adopted in several countries. Three quarters of the studies included in the review were conducted in Europe or North America, and over 90 percent were published after 2010.
MCRF has conducted annual studies of influenza vaccine effectiveness for the past 12 years. There was no funding source for this study.
A copy of this paper can be found at http://www.thelancet.com/journals/laninf/article/PIIS1473-3099(16)00129-8/abstract.