A new review of published studies finds that the seasonal flu vaccine is effective at protecting the over-60s during epidemics or widespread outbreaks.

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For people over 60, the flu vaccine is effective at protecting them during epidemics or outbreaks, according to the latest study.

Senior investigator Edwin Van den Heuvel, a professor in the Department of Epidemiology at the University of Groningen in the Netherlands, and colleagues report their findings in The Lancet Infectious Diseases.

Although anyone can get the flu, the most vulnerable groups are young children under the age of 2, the over-60s and people who are already ill with another condition.

For their review, the researchers only included case-control studies of "test-negative design." These have emerged in recent years as the preferred method for assessing the effectiveness of flu vaccines because they are thought to rule out bias due to misclassification of infection and confounding by health care-seeking behavior.

This means the researchers only included studies that compared older people with similar health care-seeking behavior. For example, people less likely to visit the doctor are also less likely to get a flu shot, so including them in the comparison sample could bias the results to make the vaccine look more effective than it is.

Dr. Michael L. Jackson, assistant investigator with Group Health Research in Seattle, WA, who writes an accompanying commentary on the study, told Reuters Health that often, people who are old and frail are less able to visit their doctor and more likely to have complications if they do fall ill. Comparing them to healthier people who get the shot is also going to make the vaccine look better than it is, he explains.

Altogether, the reviewers found 35 studies covering 53 datasets - including several from the US and Australia - that met their inclusion criteria.

Analysis showed flu shots significantly effective during widespread outbreaks

Overall, the analysis showed that during regional or widespread seasonal flu activity, over-60s who received flu shots were 28-58% less likely than others to test positive for a flu infection.

Further analysis revealed that while seasonal flu shots were not significantly effective during local virus activity, they were significantly effective during regional and widespread outbreaks, regardless of whether the vaccine was a full match to circulating viruses.

"This reinforces what we already know, the vaccine works modestly well for seniors," says Dr. Jackson.

In their conclusions, the authors note, "efforts should be renewed worldwide to further increase uptake of the influenza vaccine in the elderly population."

In July 2014, Medical News Today reported a study where researchers suggest tackling immune response as a new direction for flu drugs, rather than trying to tackle the virus itself. Reporting in the Journal of Virology, they show how such an approach might work against infection by the H7N9 avian flu virus.