Vaccines During 2009 Swine Flu Epidemic Largely Reduced Illness, Hospitalizations

Vaccines can successful prevent pandemic influenza (flu) and reduce the number of flu-caused hospitalizations, yet efficacy can vary among age groups, according to a study published inVaccine.

In 2009, a novel influenza A virus (H1N1) began appearing in humans. The virus contained a unique combination of influenza genes that had not been previously identified in animals or humans. The first case appeared in the United States in March 2009, but the new virus spread rapidly to other countries. By June 2009, the World Health Organization declared the influenza A(H1N1)pdm09 virus or “swine flu” a pandemic.

“The 2009 swine flu pandemic was the first in human history when pandemic vaccines have been available worldwide,” said lead investigator Jonathan Van Tam. “It’s therefore really important to pull all of these data together and ask the question: did these vaccines really work?”

An estimated 61 million individuals worldwide were infected with the virus, and vaccines against the new strain were developed spread across the world from September to December 2009.

Most of the availablevaccinescontained inactivated A(H1N1)pdm09 virus rather than live virus. Additionally, some of the formulations contained an adjuvant to strengthen the body’s immune response to the vaccine and allow for smaller doses of the antigen to be used.

Although individual studies have been conducted to examine efficacy of the vaccines at preventing illness and hospitalization caused by the strain, until now, no studies have summarized all the available data.

The investigators conducted a systemic review and meta-analysis to offer insight into the relative efficacy of both adjuvant and non-adjuvant vaccines in different age groups.

For the analysis, investigators used 38 studies, published between June 2011 and April 2016, that measured the efficacy of inactivated pandemic influenza vaccines that covered more than 7.6 million individuals. Of the studies, 33 reported results eligible for the meta-analysis.

In the population as a whole, the results of the study showed that the pandemic flu vaccines had a 73% efficacy in preventing laboratory-confirmed flu illness and 61% effective in preventing hospitalization.

When the efficacy of the vaccines were examined among different age groups, the investigators found that they were less efficacious in adults over 18 years than in children. The vaccines efficacy was found to be lowest in adults over 50 years.

In particular, adjuvanted vaccines had a higher efficacy in children than in adults against laboratory confirmed illness—–with 88% in children versus 40% in adults––and hospitalization with 86% in children versus 48% in adults.

Overall, the inactivated pandemic flu vaccines used in the pandemic were efficacious in preventing laboratory-confirmed illness and hospitalization. The adjuvanted vaccines tended to be more effective than non-adjuvanted vaccines but only in children, according to the study.

The authors noted that the lower efficacy in older individuals may be due to their having pre-existing antibodies against A(H1N1)pdm09 from prior exposure to a similar virus, with corresponding lower incidence of the infection in this age group.

The results of the study indicated that the pandemic flu vaccines produced globally during the 2009 and 2010 pandemic were significantly effective in reducing illness and hospitalization. The findings could be used to help public officials plan a more effective response to future pandemics, according to the authors.