A vaccine for run-of-the-mill flu also provides some protection against swine flu, especially the severest forms of the disease, Mexican scientists report.
The authors stress, however, that their study is limited in scale and that they see no evidence for dropping programmes to vaccinate against the pandemic H1N1 virus.
Investigators led by Jose Luis Valdespino-Gomez, an epidemiologist at the national laboratory Birmex, compared 60 patients who had fallen sick with swine flu with other people, matched for age and background, who were being treated for other diseases.
The patients were asked whether they had received the 2008-2009 seasonal vaccine, a cocktail of three antigens designed to prime the immune system to overlapping flu strains that circulated in the northern hemisphere last year.
The doctors looked at a subset of patients who had each been hospitalised -- 45 in the flu group and 60 in the control group.
Vaccination against seasonal flu provided effectiveness of 80 percent against catching swine flu, they found.
"This study presents clinical data suggesting that this vaccine may provide some protection," says the article, published online by the British Medical Journal.
"Moreoever, that none of the vaccinated cases of influenza A/H1N1 died indicates that seasonal vaccination might protect against the most severe forms of the disease."
The authors speculate that the H1N1 is closely related to a previous virus to which people had been exposed. Giving these people the seasonal vaccine helped prime their antibodies, the first line in the immune defences, to fight off swine flu virus.
They emphasise that wider probes are needed in order to confirm their findings.
"These results are to be considered cautiously and in no way indicate that seasonal vaccine should replace vaccination against pandemic influenza A/H1N1 2009," says Valdespino-Gomez.
The swine flu scare has cost billions in terms of economic damage and bills for formulating a specific vaccine against the virus.
As of September 27, there had been more than 340,000 laboratory confirmed cases of H1N1 and over 4,100 deaths, according to a World Health Organization toll issued last Friday.
The WHO says that swine flu is about as lethal as an ordinary seasonal virus but cautions that the pathogen could mutate into a form that would make it more virulent.