However, the researchers stress that the findings should be considered cautiously and in no way indicate that seasonal vaccine should replace vaccination against pandemic influenza A/H1N1.

For this latest study, Dr Jose Luis Valdespino and colleagues investigated the link between the 2008/9 seasonal flu vaccine with cases of influenza A/H1N1 during the epidemic in a speciality hospital in Mexico City.

They say the reason the seasonal vaccine offers some protection is because it boosts existing antibodies in individuals who have previously been exposed to a similar flu virus, either by infection or vaccination.