People vaccinated against seasonal flu appeared to have been at increased risk of the H1N1 pandemic flu that killed thousands worldwide in 2009, Canadian researchers said Tuesday.

However, they said in their findings published in the journal PLoS Medicine that the link between seasonal flu vaccinations and subsequent pandemic flu illness is tenuous.

Four studies led by Danuta Skowronski of the British Columbia Centre for Disease Control in Vancouver compared the frequency of prior vaccination in people with H1N1 influenza to people without evidence of infection.

The studies included approximately 2,700 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 percent 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 said 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 said.

Six other studies produced "highly conflicting results," peers Lone Simonsen and Cecile Viboud noted in an accompanying commentary in the magazine. Thus, it would be "premature to conclude" that seasonal flu vaccinations increased the risk of pandemic illness in 2009, they said.

The Canadian researchers also noted that the World Health Organization has recommended that H1N1 be included in subsequent seasonal vaccine formulations.

This would provide protection against H1N1 and "thereby obviate any risk that might have been due to the seasonal vaccine in 2009, which did not include H1N1," they said.