When — and for how long — is your flu contagious?

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WATCH: This is how long you’re contagious when you have the flu – Mar 4, 2018

Coming out of a flu can be a truly transformative experience. That first fever-free morning, complete with the ability to breathe through both nostrils, can be enough to make you jump for joy. And even though you may feel like you’re ready to rejoin the land of the living (and working), should you?

The answer to that is as much political as it is clinical, says Dr. Michael Gardam, an infectious diseases specialist and chief of staff at Humber River Hospital in Toronto.

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“The question of when you’re contagious has been used as justification for certain hospital policies. There’s reasonably good data about when you’re likely contagious and when you’re not, but the reality is we don’t have complete answers.”

Why is this year’s flu so much worse than previous years?

Gardam says that every flu season is unique and you never know what you’re going to get until it’s over, but there are some mitigating factors that have contributed to making this year’s a doozie.

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For starters, the flu vaccine hasn’t been very effective at protecting elderly people against influenza A (or the H3N2 virus), which is causing a more serious disease in them. Couple this with the fact that the influenza B virus made an early appearance — normally it would rear its ugly head in February, but it’s been around for the last couple of months — and it’s hanging around even though it already peaked, and you have what Gardam calls a “double whammy.”

Adding to this is the fact that it is peak RSV (respiratory syncytial virus) season, which is clinically the same as the flu, and there are three things going on at once that are working to make this an especially intense year for the flu.

The weekly influenza report from Health Canada indicates that in the week of Feb. 11-17 (the most recent data to date) virus activity was still at peak level, which it had been maintaining since the beginning of the year. Detections of influenza B were greater than the A strain, but the overall activity across the country was in a very slow decline.

“Normally, we’d see influenza A peak around Christmas and then fade out, but what’s interesting here is that it hasn’t dropped off much,” Gardam says. “Influenza B normally picks up around now but it’s been around for a couple of months and as of two weeks ago, it was still climbing. We’re not out of the woods yet.”

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How is the virus spread?

This is where things get a little murky. But before the question can be answered, Gardam provides some historical background that indicates the basis upon which the contagion and its specifics have been established is flawed.

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One highly cited study takes the example of an Alaska Airlines flight that was grounded on the tarmac and held a passenger infected with the flu. The story goes that the passengers on the flight were not allowed to deplane, so after a while, the crew brought food on board and set up a buffet.

Unfortunately, the person who had the flu was seated next to the food and 72 per cent of people on board were infected. It was then concluded that the virus was airborne and that’s how it was transmitted. But Gardam doesn’t see it that way.

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There are three ways to pass a respiratory virus: through contact (an infected person coughs on a subway pole, you touch it and then touch your eyes or mouth); through big droplets (an infected person standing within two metres of you on the subway coughs in your face); or airborne (an infected person coughs, microscopic particles float through the air down the subway car and infect someone at the end).

“We know that 99.9 per cent of the influenza virus is in the big droplets, and while it does exist in the microscopic particles, it’s much less,” he says. “If you play the odds, the big droplets are where the money is.”

Transmission through contact (the coughing on the subway pole scenario) is known to transmit rhinovirus and other viruses, but doctors don’t know for sure that it also spreads the flu. It is reasonable to assume that through that contact transmission has a role, though it’s not known how much.

By that rationale, the many people on the Alaska Airlines flight who got sick would have had to have been within two metres of the infected passenger.

“This likely happened as the sick patient was in a seat beside the food the airline had brought in,” Gardam says. “Despite the fact that this outbreak has been held up as proof of airborne transmission of influenza, we really can’t say how it was spread to other people on the plane — it is quite possible that contact, droplet and maybe airborne transmission all played a role.”

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What does this mean for the window of contagion?

Health agencies are adamant that the flu virus is contagious even before you show symptoms and Gardam agrees — to a degree.

READ MORE: Severe flu and the chance of death: How to tell if you need to see a doctor

“It’s been shown with people who have volunteered to be infected with the virus that they start to secrete it roughly 12 hours before they have any symptoms. But how infectious it is no one knows.”

More importantly, he says, whatever you are secreting at that stage is far less contagious than what you’re capable of passing on 24 hours later.

“For the five to seven days that follow, an infected person is secreting 10,000-fold more [of the contagious virus] than before,” he says.

“Some people say that [curbing] asymptomatic transmission is really important. My controversial take is: can you transmit some of it before the onset of symptoms? Yes, it’s possible. But that’s probably a very small percentage of the overall transmission.”

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His rule of thumb is to wait until your sneezing and/or hacking cough has subsided before going back to work. More importantly, don’t allow the presence or absence of a fever to dictate whether you’re well enough to go out again because that’s been shown to be an inaccurate indicator of how contagious you may be.

“At the end of your illness, you have far less chance of providing a route to easy transmission. When you’re not sneezing or coughing, it’s reasonable to assume you’re spreading less of the virus and you are secreting a lot less of the virus as well.”

He’s especially eager to disseminate this information to people in the healthcare world who, he says, are known for going to work even when they’re very ill.

READ MORE: The flu season in Canada is getting pretty nasty; here is everything you need to know

“If I can get people to stay home for even three days, that would be a massive win and would decrease flu transmission enormously,” he says. “If you’re acutely ill, you shouldn’t be at work because you could still be shedding the virus for another two to three days after peak illness.”

In other words, give yourself at least three days to get over the hump of your flu, and wait for your sneezing and coughing to subside before going back to work.


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