The results of a recent study published in the Journal of Infectious Diseases suggest that influenza vaccines, though preventive of the disease, have little to no effect on its severity.

A team of researchers at the Centers for Disease Control and Prevention, together with health departments and universities across the country, including Yale, discovered that in the age group of adults over 50 years old, influenza vaccines have no impact on the severity and complications that may result from the disease. The study was published in the Journal of Infectious Diseases on March 27.

“I think that we have some modest evidence from this one influenza year that disease severity might be reduced in a small way, in a modest way,” professor of preventive medicine at the Vanderbilt School of Medicine William Schaffner said. “But we could not detect using our methods a really substantial influence for impact on severity, which frankly surprised us.

The study collected data through the Influenza Hospitalization Surveillance Network over the 2012–13 flu season. The network included data from more than 3,000 patients 50 years or older who had been vaccinated more than four days prior to hospitalization. Schaffner added that the researchers targeted the above 50 age group because older people are particularly susceptible to influenza and especially more severe forms of the disease. Previous studies have suggested that the vaccine is effective in reducing the severity of the disease, but the results of this study stand in contrast to those data, Schaffner said.

The flu causes roughly 30,000 deaths every year in the United States, said Akiko Iwasaki, Yale professor of immunobiology and molecular and developmental biology, who has conducted research on influenza and who was not affiliated with the study. Iwasaki added that up to 90 percent of these deaths are caused by complications of influenza, such as pneumonia, and occur among people 65 years or older.

Because of the ever-changing nature of the virus and its widespread effect on the population, the annual epidemics of influenza have substantial social and economic consequences, added Schaffner.

Arthur Reingold, professor and head of epidemiology at University of California at Berkeley and co-author of the study, said in an email that each year, researchers attempt to assess the effectiveness of vaccines used against various sub-groups of the population. However, the fickle nature of the flu virus itself is what makes it difficult for researchers to create a perfect vaccine.

“Because the flu virus is constantly changing, our current approach to making flu vaccines involves making an educated guess every February regarding which strain(s) will circulate and cause disease the following winter,” Reingold said.