For the Grote Griepmeting (large flu monitoring) study, which gathered data from thousands of Dutch and Belgians for the last two years, consumers are asked to fill in a questionnaire that asks questions about their lifestyle, food consumption and overall health.
But while the latest set of data, announced last week, links vitamin use to a 1.1 higher risk ratio with flu incidence, they cannot confirm a negative impact from taking supplements, as suggested by some Dutch media reports.
They are more likely to suggest that people with an unhealthy lifestyle tend to use supplements to counter their bad habits, said Karin Postelmans, the biologist responsible for translating the results.
"It's possible that people take these supplements to compensate for bad habits like smoking," she told NutraIngredients.com.
The researchers are currently trying to better understand the link through an analysis of their data. But Postelmans said the survey may need to add further questions to gauge the kind of supplements people are taking as well as other lifestyle behaviours.
She added: "A more at-risk group are men with a lot of chronic disease and extrovert character."
These participants had a risk of 1.39.
Stress was also clearly linked to increased susceptibility to illnesses like the flu.
Men and women suffering from stress were a third more likely to come down with fever than more relaxed people, the study found.
The survey is now entering its third year and has around 30,000 participants. Portugal has also joined the project.