TORONTO - A half-dose of flu vaccine is nearly as effective as a full dose in healthy adults, particularly women, according to findings of a new study published Tuesday.
The authors suggest public health authorities might want to consider using a half-dose for healthy adults as a way of stretching supplies if seasonal flu vaccine shortages occur in the future.
But one of the authors admitted that vaccine surpluses, not shortages, are a more likely development at this point.
Dr. John Treanor, an influenza vaccine expert at the University of Rochester Medical Centre in Rochester, N.Y., said the study was started in the aftermath of the 2004 flu vaccine shortage in the United States, when nearly half the country's vaccine supply was withdrawn after it was revealed to be contaminated with bacteria.
"We're really not in the same scenario we were ... when we had the shortage and this sort of came up as a potential emergency solution," said Treanor, chief of the division of infectious diseases.
"I don't anticipate we'd ever really be faced with that again. And that's why I'm not sure it's really all that relevant anymore."
The loss of 46 million doses of vaccine in the fall of 2004 triggered a fevered reaction in the American public. People who had largely ignored flu shots in the past stood in line for hours to try to get the vaccine that was available. Thousands of Americans even crossed into Canada, where there was no shortage of supply.
The 2004 supply problem was the second time in four years the U.S. faced a massive shortfall in flu vaccine.
Those events, combined with concerns over the potential for a flu pandemic, prompted health authorities to both increase the number of companies licensed to supply vaccine and to create incentives for manufacturers to develop or expand flu vaccine-making facilities in the United States.
In the past few years, the problem hasn't been undersupply but oversupply of flu vaccine.
Whereas in 2004 there were only two licensed suppliers for the U.S. market, five companies now supply the United States. One of those companies, GlaxoSmithKline, sells two flu vaccine products into the U.S. market, from a plant in Ste-Foy, Que., and from Rixensart, Belgium.
Canada has also increased the number of licensed suppliers in this country as an insurance policy against supply problems.
Concerns over supply also spurred research, like the half-dose study, to look at ways of stretching out vaccine in future periods of shortage.
The study was led by Dr. Renata Engler of Walter Reed Army Medical Center in Washington; the findings were published in the Archives of Internal Medicine.
Engler and her colleagues showed that healthy adults, particularly those in the 18-to-49 age group, who had received flu shots in previous seasons generated similar levels of antibodies with a half-dose shot as did adults given the full dose of vaccine.
And the group that got a half-dose didn't seek medical care for respiratory illnesses at a higher rate than those who got a full dose, the study found.
Women had higher antibody responses than men. In fact, women who received a half-dose of vaccine generated the same level of antibodies as did men who got a full dose.
Treanor said this phenomenon of a stronger immune response in women has been seen before. He suggested if one needed to find ways to stretching flu vaccine, giving lower doses to women might be an option.
But he noted that public health officials have a mantra when it comes to mass vaccination programs: Keep it simple. Anything that requires one dose for this group and another for that group adds unwelcome complexity to a job that is already tough - getting millions of people vaccinated in a matter of weeks every fall.
Treanor also said the findings probably don't have any relevance for those planning for a future flu pandemic. The people in this study had all been previously immunized against flu, so their immune systems were "primed" to recognize seasonal flu strains.
"I don't think you could really extrapolate from this to anything about pandemics," said Treanor.
By definition, a pandemic strain would be one to which people hadn't previously been exposed. It is thought that people would need at least two shots of vaccine to protect against a pandemic strain.
