The inactivated monovalent influenza A(H1N1) vaccines used since the 2009 influenza pandemic, in both the adjuvanted and unadjuvanted forms, were found to be effective in the prevention of influenza and influenza-related hospitalization, according to an international review article published recently in Vaccine.1
The investigators, led by Louise E. Lansbury, PhD, from the Health Protection and Influenza Research Group at the University of Nottingham in the United Kingdom, identified 38 studies published between June 2011 and April 2016 that met inclusion criteria for meta-analysis. Data for evaluation comprised 7,643,738 patient records from multiple different study designs, including test-negative studies (n=13), case control trials (n=10), prospective cohorts (n=7), retrospective evaluations (n=4), screening-method studies (n=2), and systematic reviews (n=2).2-39
The influenza A(H1N1) vaccine was the first to be used internationally in the containment of a pandemic. Overall, the authors found both pandemic vaccines to be “broadly effective” in preventing viral infection and associated hospitalization.
The adjuvanted vaccines were the only type studied in children, where they showed greater efficacy compared with in adults. Pooled adjusted vaccine efficacy in 1126 patients from 2 studies was 86% (95% CI, 67%-94%; P <.00001).8,20 “Adjuvants are immune potentiators, which have been used for decades to enhance, accelerate, and prolong the body’s immune response to vaccine antigens,” Dr Lansbury explained to Infectious Disease Advisor.
Pooled responses to the 2 vaccines indicated only moderate protection of 49% efficacy in 5 studies of people older than 18 years (95% CI, 13%-71%; P =.01; n=3979),2-4,7,24 and neither type reached significance alone. In people older than 50 years, the pooled estimated vaccine efficacy for 2 studies using adjuvanted vaccine was 46% (95% CI, −17% to 75%; P =.22; n=1149).3,7
“Our finding that the pandemic vaccines were not as effective in older people is basically what we see with seasonal flu vaccines in the elderly,” Dr Lansbury reported. She went on to explain that “there were 2 possible reasons for this: 1) That older people had encountered a similar virus to the pandemic H1N1 strain when they were younger, so they were not immunologically naive to the new strain; and 2) that the vaccines simply don’t work as well in older people with less fit immune systems.”
She added, “As we did not see a difference in the effectiveness of unadjuvanted vaccines between adults and children, our results suggest that the adjuvants used in the vaccines in the studies analyzed provoked a stronger immune response in children.”
As previous studies have concluded,40-42 targeting children for pandemic vaccination efforts is a valid strategy.
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This article originally appeared on Infectious Disease Advisor