Just when you thought the issue of seasonal and pandemic flu shots couldn't get any more confusing, European researchers are questioning whether it makes sense to vaccinate little kids against seasonal flu.
The scientists, from the Erasmus Medical Center in Rotterdam, the Netherlands, argue preventing small children from being infected by - and developing immune responses to - seasonal flu viruses might make them more vulnerable when a flu pandemic rolls around.
It's not clear the theoretical concern they raise is true. But other scientists counter that even if it is, it makes more sense to try to protect children from a threat they face every year than one they might face every few decades.
"The simple question is should we let young children suffer from a severe and potentially fatal but easily preventable illness, just because there is a theoretical possibility that withholding vaccination might result in a slightly less severe illness sometime in the future?" a pair of Finnish researchers retorted in a counterpoint published with the perspective piece, in the journal Lancet Infectious Diseases.
"We believe that the answer to this question is a simple one."
The argument made by the Dutch researchers may actually support a concern raised by some Canadian scientists about a possible link between getting seasonal flu shots and catching swine flu.
The research, which has yet to be published, is based on data from four provinces that shows higher rates of mild H1N1 infections among people who got a flu shot last year.
The purported link, which hasn't been seen in data from several other countries, was first found by flu epidemiologist Dr. Danuta Skowronski of the British Columbia Centre for Disease Control, who noticed it in data from that province.
The finding led most provinces to delay seasonal flu shot programs until after pandemic vaccination efforts have been completed.
Internationally, though, the suggested link has been met with skepticism and has been dismissed by some as "the Canadian problem." Even the World Health Organization, which organized a teleconference of experts to explore the issue, has said the consensus of those experts was that the finding was probably due to a problem with the data, not a real risk.
But the Dutch scientists, who don't refer to the Canadian research in their article, say giving kids aged six months to 59 months flu vaccine prevents them from acquiring a broad immune response to flu viruses that can only be induced by infection.
Vaccines trigger development of antibodies to the proteins on the surface of flu viruses, proteins that change often to evade the immune system. But when you catch the flu, something called cellular immunity also kicks in. It teaches the body to recognize other parts of a flu virus; those parts don't change as much from one virus subtype to another.
Without that cellular immunity, the Dutch researchers say, children who experience a pandemic could have more difficulty fighting off the new flu strain. They suggest this may explain why the majority of fatal cases of H5N1 avian influenza have occurred in children and young adults.
They also report that they tested the theory in mice, some of which were vaccinated against the H3N2 strain of flu and others of which were experimentally infected with the same virus.
The mice were then exposed to the highly lethal H5N1 virus. The vaccinated mice died but the mice that had been previously infected with H3N2 suffered milder disease.
"This kind of phenomenon would be a potential explanation for what Danuta saw," said flu vaccine expert Dr. John Treanor, from the University of Rochester in New York State.
"The argument would be that if you walked into a pandemic and you didn't have any of those background cellular responses, you might have more severe illness or more prolonged illness than if you had previously experienced a lot of flu infections."
But Treanor, who was not an author of either of the perspective pieces, said in actual fact flu shots don't prevent infection entirely. Instead, they arm the body to nip infection in the bud before it progresses to illness.
"The vaccine is not designed to prevent you from being infected, it's designed to prevent you from getting sick. And that's what it does," he said.
And if flu vaccines were effective enough to prevent infection in the first place, they should be used to protect little kids against seasonal flu, he added.
"There's no reason to sit through getting the flu and becoming sick and possibly dying just so that you might have some cross-protective cellular immunity in the event that a pandemic occurs some day," said Treanor.
The Finnish scientists, Dr. Terho Heikkinen and Dr. Ville Peltola, of Turku University Hospital, further caution against putting too much weight on findings of vaccine work done in mice.
"It is obvious - and supported by the history of medical research - that the results of experimental animal studies can never be extrapolated directly to human beings, let alone form the basis of any vaccination policy," they say.
The Dutch researchers, led by Dr. Guus Rimmelzwaan, say more research is needed to see if their theory is true.
But they also suggest young children who live in areas where H5N1 viruses are endemic in poultry should not be vaccinated against seasonal flu, because blocking the development of cross-protective immune responses in these kids could raise the risk they face if they become infected with H5N1.
