A report on a UK survey suggests that vigorous exercise may help reduce the risk of catching the flu. The survey finds no such link with moderate exercise. However, the report authors stress the results are preliminary and should be treated with caution.

The findings come from the UK Flusurvey, in which more than 4,800 people have so far taken part this year. The online survey, which is now in its fifth year, is run by the London School of Hygiene & Tropical Medicine.

The release is geared to coincide with National Science & Engineering Week, an annual event run by the British Science Association that aims to encourage more young people to engage in science.

Dr. Alma Adler, Research Fellow at the London School of Hygiene & Tropical Medicine, who is also an ambassador for National Science & Engineering Week, says:

"We're really interested in the preliminary findings around fitness activity and flu-like illness, as exercise is something that everyone can do to reduce your chance of having flu."

She emphasizes the need to treat the preliminary findings cautiously but also notes they are consistent with findings for other illnesses, adding to evidence about the health benefits of exercise.

Vigorous versus moderate exercise

The survey results suggest doing vigorous exercise for at least 2.5 hours a week can reduce the chance of experiencing a flu-like illness by around 10%.

A news release from the London School of Hygiene & Tropical Medicine notes that based on the data they have analyzed from more than 4,800 people who have completed the survey so far, the findings:

"... suggest that 100 cases of flu per 1,000 people could be prevented just by engaging in vigorous exercise. No differences were found in rates of flu-like illness based on the amount of moderate exercise reported..."

Health experts define vigorous intensity aerobic exercise as exercise that raises your pulse rate, makes you sweat and also makes you breathe hard and fast, to the point where you cannot say more than a few words without pausing for breath. Fast running or cycling is a good example.

Moderate intensity aerobic exercise raises your pulse rate and makes you sweat, but you are not working so hard that you cannot talk or sing the words to a song at the same time. Gentle jogging and walking fast enough to break into a sweat are good examples.

The Flusurvey

The Flusurvey is an online system for tracking flu trends in the UK.

Launched in 2009, in the middle of the swine flu epidemic, the survey is now in its fifth year. Unlike traditional surveillance systems that collect data via doctor surgeries and hospitals, it collects data directly from the public who register online. The idea is to include people who do not visit the doctor - and who, as a result, do not feature in traditional flu tracking systems.

Every year, more questions are added to try and track as much information as possible about who does and who does not fall ill with the flu.

Once they register, participants are asked to fill in a profile survey asking general questions about themselves and flu risk factors (including age, vaccination status and household size). One of the questions also covers how much and what type of exercise they do, such as running, cycling and other sports.

Then, each week, participants report any flu-like symptoms since the last time they visited the site.

Lowest report of flu-like illness in recent times

This year's Flusurvey also shows some of the lowest reports of flu-like illness in recent times.

Over the winter flu season, only 4.7% of reports were positive for flu-like symptoms, compared with 6.0% last year, say the researchers who analyzed the results.

Another change they noticed is that children appear to have lower levels of flu-like symptoms compared with last year. This year, just 5.0% reported symptoms, compared with 7.9% in the year before.

The researchers suggest this could be a reason why flu has not been so rampant this year, as children are among the biggest spreaders of the disease.

In 2010, Medical News Today reported advice published by experts with the American College of Sports Medicine, who said while regular moderate exercise can help lower risk for respiratory infections, they do not recommend working out while sick, and in some cases, prolonged intense exercise can do more harm than good and weaken the immune system.