U.S. scientists believe they have found a way to almost magically quintuple the supply of the flu vaccine.
According to the research, which was rushed to press, it is possible to "stretch" the supply by diluting the vaccine and injecting it differently -- into the skin instead of into muscle.
According to two studies that will be published in the New England Journal of Medicine later this month, using 20 to 40 per cent of the full dose of the vaccine offers the same levels of protection against influenza viruses -- at least for healthy adults. An added bonus is that it hurts less.
The catch, however, is that the technique was not tested on people over the age of 60, who are at the greatest risk of contracting influenza.
Robert Belshe, director of the Center for Vaccine Development at Saint Louis University in Missouri, said that while the method could theoretically "stretch" the limited flu vaccine supply by 2½ to 5 times, it will not solve the vaccine shortage in the United States because it is too experimental.
But the information provided by the study could prove useful for future vaccination campaigns and in the eventuality of a pandemic outbreak of influenza, he said.
"We are learning that the same size dose doesn't seem to fit all ages," Dr. Belshe said.
"Younger people may need less vaccine, and older people may need more vaccine."
One of the studies had 238 patients in the United States and the other 100 patients in Belgium, all of them between the ages of 18 and 60. In both research projects, about half the participants received the shots into the muscle, and half intradermally.
The doses ranged from 20 to 40 per cent of the regular vaccine but were just as effective at producing an immune response, at least in younger people. (The study did not determine how many actually contracted the flu despite the shots; the vaccine is normally 70- to 90-per-cent effective.)
Influenza vaccine is usually injected into the muscle of the upper arm. Intradermal vaccination is an injection between the layers of the skin with a tiny needle.
The theory is that because there are more antibody-producing cells in skin than in muscle, lower doses can produce the same levels of immune response, at least in those with healthy immune systems. It is not clear whether this will hold true for those over the age of 60 who, generally, tend to have weaker immune systems.
The United States was expecting to vaccinate approximately 100 million people against influenza this year, but it has managed to secure only about 61 million doses. The shortage came about because of a production problem at the plant of a major vaccine supplier.
While the U.S. recommends that only those at high risk be vaccinated -- people over 65, and those with chronic health conditions such as heart disease and asthma -- there is a scramble to meet the demand. Many Americans have turned to Canada, where there is normally no lack of the vaccine, but the outside demand may create shortages domestically.
In Canada, it is recommended that everyone over the age of six months be vaccinated against the flu annually.
Ontario is the only province to offer the vaccine free of charge to all its residents. Other provinces provide it free of charge to people considered at high risk. The majority of provinces 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 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.