Receiving a flu shot annually markedly increases its effectiveness, particularly among those who are at high risk, such as the frail elderly living in nursing homes, according to new research.
The study, published in today's edition of the Journal of the American Medical Association, found that getting a shot has only a negligible effect the first year.
But consistently getting a flu shot reduces the risk of dying by about 15 per cent a year, and by 28 per cent during flu seasons when the virus is particularly virulent, researchers found.
Dr. A.C.G. Voordouw of Erasmus Medical Centre in Rotterdam, the Netherlands, said the research strongly supports the notion that everyone over age 65, and those working with the elderly in nursing homes and long-term care facilities, should get a flu shot every year.
He said the data demonstrate that, as a public-health initiative, flu vaccination provides real bang for the buck.
The new research suggests that for every 302 flu shots that are given, one death is prevented, and for every 195 revaccinations, one death is prevented.
The study was conducted on more than 26,000 people over the age of 65 living in the Netherlands. Subjects were followed over a four-year period. During that time, almost 3,500 participants died.
The elderly Dutch population that was studied had a very high rate of vaccination, ranging from 64 per cent at the outset to 74 per cent by study's end.
Dr. Voordouw and his team found that those who were vaccinated annually were not only less likely to die of the flu, but less likely to die of all causes. (Those who get the flu are at greater risk of suffering from other health problems that come from being hospitalized and having a compromised immune system.)
The research does not explain why regular vaccination bolsters protection but the most likely explanation is that people are protected against more strains of the virus.
The influenza virus mutates constantly but the vaccine protects against the three most-common strains in any given year.
This year, for example, the vaccine protects against A/New Caledonia, A/Fujian, and B/Shanghai.
In Canada, it is recommended that everyone over the age of six months get vaccinated against the flu annually. Ontario is the only province to offer the vaccine free to all residents.
All other provinces provide it free of charge to people considered at high risk -- those over age 65 and those with chronic lung and heart disorders. The majority of provinces also provide the flu vaccine free to children. Many employers stage free flu-shot clinics in the workplace.
In a normal year, influenza kills 4,500 to 9,000 Canadians, and more than 500,000 people worldwide.
Health Canada estimates that a flu pandemic -- meaning a new strain against which people have little immunity -- would sicken almost 11 million Canadians and kill as many as 58,000 within a matter of weeks.