People vaccinated against seasonal flu appear to have been at increased risk of the H1N1 pandemic flu that killed thousands worldwide in 2009, Canadian researchers report.
But some researchers say the study contradicts other findings and suggest the link would be difficult to prove.
The findings appear in the journal PLoS Medicine.
Four studies, led by Dr Danuta Skowronski of the British Columbia Centre for Disease Control in Vancouver, compared the vaccination history of people with H1N1 influenza to people without evidence of infection.
The studies included approximately 2700 people with and without H1N1.
The first study "confirmed that the seasonal vaccine provided protection against seasonal influenza, but found it to be associated with an increased risk of approximately 68% for H1N1 disease."
A further three studies also found an "increased likelihood of H1N1 illness in people who had received the seasonal vaccine compared to those who had not."
The researchers say these do not reveal a "true cause-and-effect relationship" between seasonal flu vaccination and subsequent H1N1 illness.
The observed association may also be "due to differences in some unidentified factor(s) among the groups being studied," they say
Previous studies, including one by Dr Heath Kelly and Kristina Grant of the Victorian Infectious Diseases Reference Laboratory in Melbourne, have produced conflicting results.
In an accompanying commentary appearing in PLoS Medicine, Dr Lone Simonsen and Dr Cecile Viboud say it would be "premature to conclude" that seasonal flu vaccinations increased the risk of pandemic illness in 2009.
Dr Ian Barr, Deputy Director of the WHO Collaborating Centre for Reference and Research on Influenza in Australia agrees.
"It's not clear in my mind what the real outcome is," he says. "I would tend towards the neutral."
According to Barr, previous research on animal models has shown no beneficial or detrimental effect from the seasonal vaccine on H1N1 response.
Barr says the number of H1N1 infections this year is expected to be lower than in 2009, due to a smaller "reservoir" of people without immunity.
"We have reasonable evidence that people above the age of 50 years have existing immunity."
A large number of Australians were infected with the virus, with between 40% and 50% of children exposed, he adds.
The Canadian researchers also note that the World Health Organization has recommended that H1N1 be included in subsequent seasonal vaccine formulations, which has been included this year in Australia.
The Canadian researchers believe this would provide protection against H1N1 and remove "any risk that might have been due to the seasonal vaccine in 2009, which did not include H1N1."