According to the findings of a case-control study published in the BMJ, contrary to earlier speculation, some level of cross protection against the new influenza A/H1N1 virus may be provided by the 2008-9 seasonal flu vaccine.

The study was conducted in a Specialty hospital in Mexico City from March to May 2009 and involved 60 patients with laboratory confirmed influenza A/H1N1. They were compared with 180 controls with other diseases (not influenza-like illness or pneumonia) living in Mexico City or the State of Mexico, matched for age and socioeconomic status. The aim was to evaluate the association of 2008-9 seasonal trivalent inactivated influenza vaccine with cases of influenza A/H1N1 during the influenza epidemic. The main outcome measures was effectiveness of trivalent inactivated influenza vaccine against influenza A/H1N1.

The following findings were reported:

• The proportion of patients who died among vaccinated cases was lower than among unvaccinated cases (0/8 (0%) v 18/52 (35%), p=0.047).

• The proportion of patients requiring invasive mechanical ventilation was lower among vaccinated cases than among unvaccinated cases, although this was not statistically significant (1/8 (13%) v 25/52 (48%), p=0.058).

• With the exception of the age group 5-20 years, the crude odds ratio for vaccination during the previous season showed a protective effect for all age groups, although this was not statistically significant.

• Vaccination status and underlying conditions were independently associated with influenza A/H1N and when the association of influenza A/H1N1 with vaccine status for each age group was modelled, the adjusted odds ratio continued to show a protective effect, although this was statistically significant only for the age group 41-60.

• Vaccine effectiveness against laboratory confirmed cases of influenza A/H1N1 was 73% .

The researchers conclude that these data suggest the seasonal flu vaccine may provide some protection against influenza A/H1N1, diagnosed in a specialty hospital during the epidemic in Mexico, They caution that these results in no way indicate that seasonal vaccine should replace vaccination against pandemic influenza A/H1N1 2009, but rather “support the hypothesis that partial protection provided by the seasonal vaccine may be explained by the boosting of existing antibodies that were elicited by previous exposure, through either infection or vaccination, to an influenza A/H1N1 virus genetically and antigenically more closely related to the novel influenza virus than contemporary seasonal H1N1 strains.” They note that “the impact of seasonal vaccine over protection against pandemic influenza induced by pandemic influenza vaccine is presently being studied in different clinical trials by the National Institutes of Health and other agencies.”

According to an accompanying editorial, “antibodies that recognise multiple influenza strains are rare, which explains the frequent lack of cross protection between vaccines and natural infection. However, antibodies have been identified that may open possibilities of developing a "universal flu vaccine.”