Doctors have long extolled the flu shot's importance, prompting many to wince through the needle every year. But a new analysis has cast doubt on the effectiveness of that annual ritual.

Vaccines provide a moderate shield against pesky influenza viruses but that protection can get weaker or disappear entirely during some seasons, according to a report published in The Lancet.

"While the vaccine does work and we still recommend that it be used, it does not demonstrate the kinds of efficacy levels we have reported," said the report's lead author Michael T. Osterholm.

Even TIV -- the most widely-used flu vaccine in the United States -- was only 59 per cent effective in healthy adults, the analysis found.

Osterholm and colleagues from the University of Minnesota reached their conclusion by sifting through 31 comprehensive studies on the efficacy of the flu vaccine.

While the report focuses particularly on vaccines used in the United States, Osterholm notes the analysis also reviews vaccines circulating in other parts of the world including the 2009 global pandemic vaccine.

Part of the problem, the report found, is that the medical community lacks evidence of how effective the flu shot is for some of the most vulnerable members of society.

"We didn't find any trials that actually provided us with what we thought was state-of-the art information in children aged two to 17 or adults over age 65," said Osterholm.

Children and the elderly are considered the most susceptible to flu-related illnesses and even death, he notes.

To remedy these gaps, the report calls for "new generation of more highly effective and cross-protective vaccines."

Approaching flu season

To promote the vaccine, Health Canada has dubbed October "Influenza Immunization Awareness Month." The lengthy moniker is based on estimates that flu season runs from November to April.

Recent figures from the Public Health Agency of Canada show there have been no flu outbreaks reported in Canada yet, just sporadic cases.

Meanwhile, Minister of Health Leona Aglukkaq has urged all Canadians over the age of six months to head to a local health centre and get the flu vaccine.

Every year, between 2,000 and 8,000 Canadians die of the flu and associated complications, according to Health Canada.