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The researchers found that the universal vaccine program prevented about 61 per cent of flu cases each season.

Immunizing the entire population against the flu prevents more than 60 per cent of cases and makes more economic sense than singling out high-risk groups for protection, a new study concludes.

"Compared to a targeted influenza immunization program, the universal program reduces influenza illness attack rates, morbidity and mortality at reasonable cost to the taxpayer," said Beate Sander, a researcher in the division of clinical decision-making and health care research at the University Health Network in Toronto.

The research, published in today's edition of the journal Public Library of Science (PLoS) Medicine, examined data from Ontario, which began offering flu vaccine free-of-charge to all residents in 2000.

To date, Ontario is the only province that has a universal seasonal flu vaccine program. (By contrast, every jurisdiction offered H1N1 influenza vaccine to residents.)

While Ontario's program is universal, only 42 per cent of the population actually gets immunized. In other provinces, which tend to offer the vaccine to those over 65 and those with respiratory illnesses, the vaccination rate is 28 per cent, according to background information in the paper.

Ms. Sander and her colleagues at the University Toronto, where she is a doctoral student, found that the universal program costs about $40-million a year, which includes cost of vaccine, promotion and physician compensation.

They compared that program to one that targeted high-risk groups like the elderly and those with chronic illnesses such as chronic obstructive pulmonary disease (COPD), which would cost about $20-million.

The conclusion, broadly speaking, was that the additional costs are offset by the savings that result from fewer people getting sick and seeking medical attention for influenza symptoms.

The researchers found that, on average in Ontario, the universal vaccine program prevented:

* Almost 35,000 cases of the flu, or about 61 per cent of cases each season;

· 111 deaths, a 28 per cent reduction in mortality;

· 786 influenza-related hospitalizations, a 46 per cent drop;

· 7,745 emergency room visits for flu treatment, or 61 per cent fewer visits;

· More than 30,000 visits to doctors for treatment, also a 61 per cent drop.

Overall, the universal vaccine program cut treatment costs by just over half, the researchers found.

Ms. Sander said the figures almost certainly underestimate the benefits of vaccination because "a lot of people who get sick with the flu don't seek medical attention" and the calculations do not include the economic benefits that come from workers not taking sick days and children not missing school.

She stressed, too, that the numbers are averages, based on data from three years before the introduction of universal vaccination and four years after.

"Some seasons are mild and some are severe but you can't know that in advance," she said in an interview. "But, on average, it's cost effective to have a universal program."

The principal critique of universal flu vaccine programs is that they tend to attract healthier members of the population.

Ms. Sander conceded that is true but said "healthy people transmit the flu, too."

Asked if other provinces are making a mistake with targeted vaccination programs instead of universal plans, Ms. Sander responded cautiously, saying: "I don't think it's necessarily a mistake. But our economic evaluation suggests the relationship between costs and health benefits make universal vaccination economically attractive."

Between 4,000 and 8,000 people die of influenza in Canada each year, according to the Public Health Agency of Canada.