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Marva Amour, right, receives a flu vaccine from RPN Yuvon Asare, left, during a clinic conducted by Toronto Public Health at the Scarborough Civic Centre on Jan. 9, 2013.Peter Power/The Globe and Mail

How effective is the flu shot? It depends whether you see the glass as half empty or half full.

Early reports in Canada suggest that, this year, the influenza vaccine reduces the risk of infection by 50 per cent. It's an interim figure that could change once a post-mortem is done at the end of the season. Regardless of the final rate, one thing is clear: The flu shot doesn't protect as well as many other vaccines, and researchers around the world are urgently striving to develop a better one.

A study published last year by the Center for Infectious Disease Research and Policy at the University of Minnesota found that, on average, the flu shot is 59 per cent effective for healthy adults between 18 and 59. The vaccine is less effective in seniors because their immune systems often aren't able to mount a strong enough response.

Questions over effectiveness rates are stoking debate over the value of the vaccine. Some even say it's time to stop public health campaigns, like the one in Ontario, that tell healthy young people, who have little risk of complications, to get the shot.

"When I'm advocating the vaccine, I'm mostly thinking about people who are at high risk for complications," said Danuta Skowronski, epidemiology lead for influenza and emerging respiratory pathogens at the B.C. Centre for Disease Control. "I think we have a duty to work toward getting a better vaccine, and in the meantime, I think people are correct to question and ask why isn't this vaccine performing as well? And what are you doing about it to get a better vaccine for us?"

Skowronski takes issue with messages that suggest healthy adults are irresponsible if they don't get the shot. For instance, CBC personality Rick Mercer aired a humorous rant about the importance of the flu shot on his show last November in which he said it's "just a little prick. Don't be one, get one."

"To me, that's name-calling and that's just blunt bullying, and I don't think that's right," Skowronski said, noting that vulnerable populations should still get the flu shot annually. "There's clearly room for improvement with this vaccine."

But Irene Armstrong, associate medical officer of health at Toronto Public Health, said: "We'd like it to be better, definitely we would … But you know what? It's really the best thing we have to prevent [people] from getting the flu."

So why isn't the flu shot more effective?

Nearly 100 per cent of children are immune to measles, mumps and rubella after receiving two doses of the vaccine. But with the flu, it's a much different story. The virus mutates year to year, and vaccines must be created to deal with what are believed to be the strains in circulation. The predominant flu strain that is causing illness in Canada this year is influenza A subtype H3N2.

The flu virus looks like a beach ball that's covered with tree-shaped proteins called hemagglutinin. The "leaves" of the tree lock into our cells, which causes us to develop the flu. The influenza vaccine binds to the leaves, preventing the virus from getting in.

However, it's possible that in any given year, there could be issues that affect how well the vaccine works. For instance, the vaccine may not bind effectively to the "leaves." But researchers also say they don't fully understand all of the variables involved.

"It's more complex and I don't think … we really have all the answers on that," said Carolyn Pim, associate medical officer of health with Ottawa Public Health.

But scientists do believe that targeting different areas of the virus – namely the trunk or stem of the hemagglutinin, which remains stable year to year – is the key to developing a more effective flu vaccine that can be used against all strains and can provide immunity for years.

Despite the problems with the flu vaccine, people who are at risk of potentially deadly flu-related complications such as chest infections and pneumonia – namely the elderly and people with compromised immune systems – are urged to get it every year.

"Even though influenza vaccine is not as effective as other common vaccines, 'not as effective' does not mean 'not effective,' " Thomas Talbot, chief hospital epidemiologist at Vanderbilt University Medical Center, wrote in the Journal of the American Medical Association this month.

In other words, despite its flaws, the flu shot can protect a large segment of the population which, in turn, won't pass the illness to others, including the elderly and other vulnerable people. This is the message many people aren't getting, said John Spika, director general at the Centre for Immunization and Respiratory Infectious Diseases at the Public Health Agency of Canada.

"There are two reasons to get the flu shot. One is to protect yourself, and the other is to protect your loved ones, who may be more susceptible," Spika said. "I think perhaps the discussion has been too much focused on protecting oneself."