As parents scurry to accommodate all possible shots for their wards this flu season, it could be worthwhile to understand new insights into annual influenza vaccinations.

With most countries and government recommending annual flu vaccinations for certain high-risk groups and healthy children against seasonal virus, the effect of the annual shot might not be all that favorable.

According to a new research paper published in the November Journal of Virology, vaccinating children annually against influenza virus could interfere with their development of cross-reactive killer T cells to flu viruses.

Lead author Rogier Bodewes of Erasmus Medical Center, Rotterdam, and his collaborators warn of potentially conflicting  outcomes from annual flu shots.

Annual flu vaccines are effective against seasonal flu, but could leave people more vulnerable to novel pandemics, said Bodewes. Induction of virus-specific killer T cells caused by childhood flu infection may reduce morbidity and mortality rates from pandemic influenza viruses.

Most countries recommend annual flu vaccination of certain high-risk groups to protect against seasonal influenza, said Bodewes. Furthermore, some countries recommend annual influenza vaccination of all healthy children more than six months of age.

Referring to the paper, Bodewes said that the findings highlight the need for the development and use of universal influenza A virus vaccines for children, especially in light of the pandemic threat of avian influenza A/H5N1.

Bodewes, however, concludes that efforts to develop such vaccines have for several decades been stumped by the sheer complexity of targeting inner proteins.

In the current study, researchers collected blood samples from Dutch children with cystic fibrosis, who are vaccinated annually against influenza. Blood samples were also collected from healthy control children who were not vaccinated; both sets were tested for the presence of virus-specific killer T cells.

The majority of virus-specific killer T cells are directed to conserved viral proteins, or proteins that are very similar among different flu viruses, unlike the rapidly evolving, highly variable proteins which are targets of antibodies induced by influenza vaccines.

In unvaccinated children, the investigators found that the number of virus-specific T cells rises with age. Such an increase was absent in children vaccinated annually. In fact, vaccination appeared to interfere with induction of such killer T cells, said Bodewes.

Vaccinated children with [cystic fibrosis] will develop lower cross-reactive virus-specific CD8+ T cell responses than unvaccinated children, said the study.

In another development, the Joint Committee on Vaccination and Immunisation in the UK, which advises the government on vaccination policy, has sought more data for child flu plans.

In a recent directive, UK Health Secretary Andrew Lansley has asked the JCVI to look at whether the flu vaccination program should be extended to all children. It said more information is needed on the availability of flu vaccines for children which are likely to become available in the UK.

Further assessment is also needed of the impact on GPs and schools of vaccinating healthy children, and the resources needed for such a program.

In the U.S., the Centres for Disease Control and Prevention recommends flu vaccine for all children including healthy youngsters, aged six months and older.