This year’s flu season is shaping up to be especially bad, with the current weekly number of cases at extraordinarily high levels. Meanwhile, doctors are seeing a surge in respiratory syncytial virus, more commonly known as RSV, which generally causes mild, cold-like symptoms in adults, but can be especially dangerous for very young children and the elderly. This RSV surge has already led to an unusually high number of hospitalizations, mostly in young children. These surges in flu and RSV infections are happening at an earlier time of the year, and at higher levels than is typical.
There are a lot of unknowns about why flu and RSV are at unusually high levels. However, one thing is for certain: “Just because we wore masks all this time, it didn’t mean that we hurt our immune system,” said Raywat Deonandan, an epidemiologist at the University of Ottawa. “Your immune system is not like a muscle, where if you don’t use it, you lose it.”
If the culprit isn’t that we wore masks for a full year, then what do we know about why this year’s flu and RSV season is higher than usual?
How does our immune system work?
Our immune system is being used a lot more than we realize, even if we aren’t getting sick. “Our immune system is not atrophying, it is not weakening, it is working on a daily basis,” said Sabina Vohra-Miller, the founder of Unambiguous Science. As Vohra-Miller points out, although respiratory viruses such as the flu were down in the 2020-2021 season, our immune system is constantly being exposed to pathogens that are in our food and water, the vast majority of which don’t ever result in illness.
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Our immune system also has a very long memory, where it “works like a photo system,” said Colin Furness, an epidemiologist at the University of Toronto. “It’s very permanent.” Our immune system learns how to recognize specific infectious agents that it has seen before.
As we get older, our immune system will start to decline, similar to how photos wrinkle with age. However, “in kids, and in healthy adults, if you don’t have some kind of immunocompromised health condition, those photos remain very intact,” Furness said. “It doesn’t matter that you haven’t had flu in years, your body will respond to flu as it responded the last time.”
With a virus such as the flu, it can only evade the immune system by changing to the point that it is no longer recognizable, while for viruses and pathogens that don’t change very much, such as measles or the chicken pox, the immune system will be able to fight it off the next time an exposure happens, even if it has been years.
Flu and RSV are seasonal
Although not being exposed to respiratory viruses for a year won’t affect a person’s immune system, the unusually low number of cases during the 2020-2021 season may be part of why we are seeing such high numbers of flu and RSV. “There is some truth to the idea that because of pandemic restrictions holding back all kinds of respiratory illness, we are seeing a resurgence of them, in general,” Deonandan said.
However, this resurgence has to do with the seasonality of viruses such as flu and RSV, rather than a lack of infections affecting a person’s immune system. As a number of scientists predicted in a 2020 paper, the low numbers of respiratory infections, combined with the seasonality of these viruses, can lead to higher-than-usual infections in later seasons.
Right now, with school back in session, “it really rekindled the viruses within the school environment,” said Pedro Piedra, a virologist at Baylor College of Medicine. All of these viruses circulating among school kids has the ripple effect of infecting others in their social circles, such as their parents, who then spread it to their colleagues.
COVID infections may be affecting our immune system
One factor that may be contributing to this year’s unusually bad flu and RSV seasons is the effect of a COVID infection on our immune systems. As early evidence suggests, this may be playing a role. “There are a number of papers suggesting that a COVID infection might reduce our ability to fight off future infections of a variety of types,” Deonandan said.
This is not a new idea: there are a number of viruses that are known to have a negative impact on our immune system. One example is the measles virus, which can cause our immune system to “forget” past infections. As a 2019 study showed, a measles infection can have the effect of wiping out anywhere between 11 to 73% of our body’s antibodies.
It’s not yet known how much COVID can reduce our immune system response, who may be susceptible, and what the effects may be. “A lot of people have had multiple infections who are just fine,” Deonandan said. “It’s just that there may be a proportion of individuals who get infected or reinfected, and that is compromising their ability to fight off future infections.”
To reduce your risk, employ precautionary measures
The flu causes between 12-52,000 deaths per year, and RSV causes 58,000 to 80,000 hospitalizations per year in children under the age of 5. Although this year is on track to be an especially bad one, the risks are well-established. “Respiratory viral infections, before the pandemic, during the pandemic, and after the pandemic will have a significant impact on our health,” Piedra said. “None of this is new.”
The advantage is that there are a number of precautionary measures that can reduce the risk of either getting sick, or lessening the severity of these symptoms. This includes staying up-to-date on your COVID vaccinations, making sure you get your flu shot, and employing precautionary measures, such as wearing a mask, when you are in a crowded environment.