The seasonal flu vaccine protects seniors during so-called epidemic seasons, when flu is widespread, according to a review of past studies.

It is important to reassess the effectiveness of the vaccine every year, the authors write, but based on these results, it’s a good idea for people over 60 to get a flu shot every year, they conclude.

“This reinforces what we already know, the vaccine works modestly well for seniors,” said Michael L. Jackson of Group Health Research Institute in Seattle, who wrote a commentary accompanying the review in The Lancet Infectious Diseases.

“When it’s offered in the fall is the best time to get it,” he told Reuters Health.

“Although influenza virus can affect everyone at any age, it is well-known that children younger than age two, elderly aged 60 years or older and patients with chronic underlying medical conditions are at higher risk of influenza and its complications,” said the review’s senior author, Edwin R. van den Heuvel of Eindhoven University of Technology in The Netherlands.

Almost 90 percent of all influenza-related deaths occur among the elderly population, he noted.

The flu vaccine relies on the body’s immune response, which gets weaker with age and may mean the vaccine is less effective, van den Heuvel told Reuters Health by email.

Nonetheless, past research has found that even partial protection may prevent some people from being infected or at least make their bout with flu a mild one.

To see how effectively standard seasonal flu vaccines protect older adults, van den Heuvel’s team reviewed 35 studies of people over age 60 who went to the doctor with flu-like symptoms between 2007 and 2014. All had laboratory confirmation of whether or not they had an influenza virus infection and the studies were done in many countries, including several in the U.S. and Australia.

By their design, the studies only compared older adults with similar healthcare seeking behavior – they all went to the doctor with flu-like symptoms. People unlikely to go to the doctor with an illness are also less likely to get vaccinated, and including them could lead to overestimating the vaccine's effectiveness, the authors write.

“Very frequently you have seniors who are old and frail, less able to come in and more likely to have complications of influenza if they do get sick,” Jackson said. Comparing those patients to healthier adults who do get the shot is going to make the vaccine falsely look very good, he said.

In general, during regional or widespread seasonal flu outbreaks, elderly people who had received the flu vaccine were 28 percent to 58 percent less likely than others to test positive for a flu infection, according to results in The Lancet Infectious Diseases.

The protective effect was strongest when the vaccine matched the circulating strain of the virus that year, but was somewhat effective even when mismatched. Only when flu was confined to sporadic local cases, being vaccinated seemed to have no effect, whether or not the vaccine was a good match to the circulating strain.

The vaccine was more effective during times when the World Health Organization had deemed a "flu epidemic season," the authors write.

“Based on our findings, we believe that governments, health agencies, and other institutes should try to maximize influenza vaccination uptake among the elderly population,” van den Heuvel said. “Thus our message would be that seniors should be vaccinated for influenza.”

In the U.S., seniors get the vaccine more than any other age group, Jackson noted. But about 30 percent still do not get the shot, likely due to poor health or an inability to get to a clinic for a shot, he said.