Respiratory viruses are on the rise. Did COVID-19 make us more vulnerable to other illnesses?

COVID-19 could be a "risk factor" for other viruses like flu and RSV, experts say

By Nicole Karlis

Senior Writer

Published December 14, 2023 5:29AM (EST)

Young woman with high temperature and flu lying down on sofa (Getty Images/gpointstudio)
Young woman with high temperature and flu lying down on sofa (Getty Images/gpointstudio)

If it seems like everyone is sick right now, it’s not just all in your head. According to the Centers for Disease Control and Prevention (CDC) over a dozen U.S. states are experiencing higher-than-usual rates of infections this winter. Of course, COVID-19 is on the rise again, yet another indicator that the pandemic isn't truly over. Recent CDC data reports COVID hospitalizations are up 17% and deaths are up 25% from last week. But respiratory syncytial virus (RSV) cases are also “elevated," in addition to pneumonia and flu cases. There's a lot going around right now, making a lot of people wonder, is this normal?

Similar to the urban myth with children, which is that more kids are getting sick more frequently post-pandemic, a similar belief is circulating that more people are getting more sick in post-pandemic winters. In July 2023 the CDC estimated 77.5% of the U.S. population had antibodies from at least one COVID-19 infection, meaning only 1 in 4 had yet to get the infection.

As Salon has previously reported, severe cases of COVID can trigger a hyperinflammatory response called a "cytokine storm" so intense that it seems to exhaust the T cells and decrease their number. More recently, there has been evidence to suggest this can affect the immune system fighting future infections from both COVID-19 and other diseases as well.

Previously, experts were seeing the consequences of measures that the country took to mitigate the spread of COVID-19. Social distancing, masking, school closures not only worked for the coronavirus, but they also helped prevent people from other respiratory viruses such as the flu. Indeed, flu numbers hit a record low in the 2020-2021 season. The number of children in pediatric intensive care units for bronchiolitis and pneumonia also plummeted between April and June 2020. Viruses seemingly came back with vengeance. But is it possible that COVID-19 has left us more vulnerable to other illnesses and more COVID-19 infections?

Viruses like the flu and RSV are starting to spread earlier than in pre-pandemic winters, hence the perceived increase in prevalence. 

“Having COVID is a risk factor for RSV and respiratory illness,” Dr. Rajendram Rajnarayanan, of the New York Institute of Technology campus in Jonesboro, Arkansas, told Salon, pointing to a study published last October in the journal Family Medicine and Community Health. The study found that prior COVID infections can make children five and under more vulnerable to RSV.

But Rajanarayanan said we have yet to pin down what the exact mechanism is behind this relationship. “We need to invest some more money to study what's going on.”

He emphasized this is just one study involving children. However, separate studies on adults have found that having COVID-19 can be a risk factor for Type 2 diabetes. A previous COVID-19 infection put people over the age of 50 at an increased risk for herpes. COVID-19 can also trigger an episode of high blood pressure.

“These are comorbid conditions for having other conditions,” Rajnarayanan said. “And in a way, it sets you up for failure for the next season.”

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In the 2010s, leading up to the pandemic, flu hospitalizations varied each year between 12,000 to 710,000 people. Taking a closer look, flu and pneumonia deaths in the winter of 2022-2023 were higher than in winter 2019-2020, right as the coronavirus took off. This year, according to data from early December, the CDC estimates that there have been at least 26,000 hospitalizations, and 1,600 deaths from flu so far this season.

Dr. William Schaffner, a professor of infectious disease at Vanderbilt University Medical Center, told Salon he’s seeing an uptick in influenza, COVID and RSV hospitalizations locally, in addition to common cold viruses swirling around. But it’s hard to say if post-pandemic what we are seeing is more people getting sick. Instead, he theorized, what’s happening is that viruses like the flu and RSV are starting to spread earlier than in pre-pandemic winters, hence the perceived increase in prevalence. 

“What happened last year, and to a degree this year, is that both influenza and RSV started early,” Schaffner said. “And some of them may have to do with coming out of COVID as the viruses have new opportunities to spread.”

When a person catches the flu it can weaken the immune system even further, making it more difficult for the body to fight off other infections or illnesses.

Schaffner said he doesn’t think that when a healthy person gets COVID-19, that automatically “primes” or “impairs” the immune system so that people are more prone to catch another respiratory virus. However, he pointed out that for people with long COVID, they seem to be having a chronic inflammatory response and an altered immune system. This could make them more vulnerable to other respiratory illnesses, he said. 

“But for the average person, those who recover from COVID, there seems to be no alteration in their immune system,” he said. “They still respond to other infections and other vaccines in a normal way — COVID doesn’t seem to predispose you to more viral infections.”

Instead, the issue is that there are “a lot of viruses that are going around simultaneously,” and this could be wreaking havoc on peoples’ immune systems as they’re having to work harder to fight off multiple viruses spreading around the country. 

Dr. T. Ryan Gregory, an evolutionary and genome biologist at the University of Guelph in Canada, told Salon “concurrent infections,” like having the flu and COVID-19, are worse for a person’s immune system than just having one infection.

“A recent COVID infection might provide temporary immunity to the next COVID reinfection,” he added. “But as with many other viruses, it could make people more susceptible to secondary infections for a while.”

Despite the myth that the immune system becomes weak if it’s not exposed to pathogens, suggesting that it’s good for a person’s immune system to be periodically exposed to viruses, experts have said that’s not the case. Immune systems are far more complex. For example, when a person catches the flu it can weaken the immune system even further, making it more difficult for the body to fight off other infections or illnesses.

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“At minimum, adding a third virus [COVID-19] along with flu and RSV increases the chances of a coinfection, which makes things worse,” Gregory said. “It can also increase the severity of infections with other viruses in people who have recently recovered from one of the others.”

The good news is that there are vaccines to protect against all three viruses circulating — COVID-19, influenza, and RSV for those over the age of 60. 

“Our vaccines have been underutilized this season,” Schaffner said. “It is not too late to be vaccinated — but do not delay in order to be protected through December and into the New Year.”

By Nicole Karlis

Nicole Karlis is a senior writer at Salon, specializing in health and science. Tweet her @nicolekarlis.

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Covid-19 Flu Health Immunity Public Health Reporting Rsv