Half-dose flu shots are effective in adults, especially in women and those younger than 50, and offer a viable way to stretch supplies during vaccine shortages, a government study found.
The strategy also might be an option during hard economic times since lower doses likely would mean cheaper shots, said Vanderbilt University vaccine expert Dr. Kathryn Edwards, who wasn't involved in the study. And the lower dosage could open doors to vaccinating people in poor countries where flu shots are little used, she said.
Even so, Edwards said giving half-dose flu shots isn't ready for prime time. It's still experimental and hasn't been approved by federal authorities.
The study involved 1,114 adults aged 18 to 64. It's the first to test half-dose flu shots in those aged 50 and older during a single flu season, 2004-05. The results among younger adults echo previous research, said lead author Dr. Renata Engler of Walter Reed Army Medical Center.
The government-funded study appears in Monday's Archives of Internal Medicine.
"Traditionally, vaccine programs have followed a 'one-size-fits-all' approach," Engler said. That means everyone gets the same dose and during shortages, supplies are more likely to run out.
If the study results are confirmed through additional research, Engler said, half-doses could be given to large numbers of adults, enabling more people to get vaccinated.
That's important because while influenza is often underestimated, the federal Centers for Disease Control and Prevention says each year the disease is responsible for about 36,000 deaths and 200,000 hospitalizations nationwide.
In the study, participants were randomly chosen to get full- or half-dose flu shots in late 2004 in the Washington D.C. area. The researchers measured blood levels of antibodies to flu virus before vaccination and 21 days afterward.
After the shots, similar numbers of adults of all ages, including men and women, had antibody levels considered adequate to protect against the flu.
The 18-to-49 age group and women had the highest antibody levels. That adds to evidence that women may be more sensitive to some vaccines than men.
Dr. Ronald Hershow, an infectious disease specialist at the University of Illinois's Chicago campus, noted that while half-doses produced an adequate immune response, full doses produced a stronger response. And there's evidence that stronger immune responses provide better protection against disease, he said.
Still, the study authors argued that from a public health standpoint, it would be better to vaccinate many people with lower doses than fewer people with full doses when vaccine supplies are scarce.
There were few reports of flu-like illnesses among the study volunteers, but the number of people of all ages with those symptoms was similar in both the full-dose and half-dose groups.
Engler noted that because adults in their 60s and older are more vulnerable to flu complications, more research is needed to be sure that half-doses are adequate for them.
The study was done during the vaccine shortage in the winter of 2004-05 when contamination was found at a major vaccine supplier's plant in Britain.
Now, there are five licensed flu vaccine manufacturers, making shortages in the near future unlikely. Still, Dr. Joseph Bresee, the CDC's flu chief, said the study provides useful information just in case.
Flu season starts in the fall and this year is off to a pretty typical start, with low levels of disease nationwide, Bresee said.
"It's still a good time to get a vaccine," Bresee said,