But they called for further work to prove the link was ‘real and causal'.
Canadian researchers analysed data from four studies, conducted by the country's public health agencies in response to a local outbreak in spring-summer 2009.
The researchers looked at whether 2,731 people included in the studies were more likely to contract pandemic flu if they had been given the trivalent influenza vaccine (TIV) in the 2008/9 flu season.
The results of one study showed that TIV helped protect recipients against seasonal flu. However, combined data from all four studies showed there was a 1.4- to 2.5-fold increased likelihood of pandemic flu illness among those given the seasonal flu vaccine the previous year, compared to those who had not.
The researchers noted, however, that the studies do not show whether this was a causal link or simply a methodological bias.
Authors said that because the result was contrary to established knowledge, greater scrutiny was needed to determine whether the results were real.
They added: ‘If these observations do reflect a real biological effect, however, they raise important questions that warrant further scientific investigation.'
However, the WHO has recommended the pandemic flu strain be included in the seasonal flu vaccine, which would provide protection and preclude the risks identified in the study.
In an accompanying editorial, researchers from the National Institutes of Health said: ‘Given the uncertainty associated with observational studies, we believe it would be premature to conclude that [the vaccine] increased risk of 2009 pandemic illness.'
This was in light of six other contemporaneous observational studies in civilian populations that have produced highly conflicting results, they added.