A case of viral interference?

Three years after the pandemic, Covid-19 is still gaining momentum, with successive waves of spikes in cases, subsiding, and then rising again. But last fall we saw something new, or rather an old one: the return of the flu. In addition, respiratory syncytial virus (RSV) — a virus rarely talked about in a typical year — surged, causing a “triple infection.”

Influenza and RSV mostly disappeared during the first two winters, so the surge in these old adversaries was particularly impressive. pandemicMore surprisingly, certain versions of the flu extinct During the early Covid pandemic.of the World Health Organization monitoring program has not positively detected a B/Yamagata influenza strain since March 2020. Memphis. But “I hope it’s squeezed out,” he adds. Webby says such an extinction would be a very rare event.

But then the last few years have been very unusual times for the human-virus relationship, and lockdowns and masks have gone a long way in keeping the flu and RSV from entering the human nostrils. Webby thinks another factor may have kept them at bay while Covid was raging. This is called virus interference and means that the presence of one virus can block another virus.

Viral interference can occur in individual cells in the laboratory and in individual animals and people exposed to multiple viruses. This results in waves of infection by individual viruses, which in turn gain the upper hand. “Looking back over the past few years, I am confident that Covid will definitely block the flu and his RSV,” he says.

This isn’t the first time scientists have observed such patterns. For example, in 2009, the feared virus was swine flu, which passed from pigs to humans in the spring of that year. It seemed to pick up momentum as autumn came, but suddenly it stalled in some parts of Europe.Rhinovirus, the cause of the common cold and likely spread by children returning to school, played a central role in the series of weeks before swine flu regained control. delayed the typical fall rise of RSV Two and a half months maximum.

running interference

There are many ways interference can occur within the body. One is when two viruses use the same molecule to enter the host cell. If Virus A gets there first and grabs all those molecular doorknobs, Virus B is out of luck.

Another kind of interference can occur when two viruses compete for the same resources in the cell. For example, the machinery for making new viral proteins or the means of escaping the cell and infecting other viruses. “Think of it as a race between two viruses,” he says Webby.

However, the best-understood method of interference involves defense molecules called interferons. made by the cells of all animals with backbones (and possibly some invertebrates too).In fact, viral interference Why was it named interferon First things first. When the cells sense the virus, they start making interferon. And it in turn numerous protective genesSome of the products of these genes function within cells or at their boundaries, preventing additional viruses from entering and blocking already existing viruses from replicating or exiting the cell.

Cells secrete interferon into their surroundings, warning other cells to be vigilant. The result of all this: If a second virus occurs, the cells have already activated their defenses and may be able to shut them down.

This “be careful” message can spread throughout your body. In theory, therefore, infection with a respiratory virus such as rhinovirus could activate defenses such as the gut, protecting the same person from an entirely different virus. norovirusHowever, circumstances vary depending on the virus involved, the amount of interferon produced, and other factors. “Most viruses themselves have ways to neutralize the interferon system,” says Ganes Sen, a virologist at his clinic in Cleveland, Ohio. interferon and virus for Annual Review of Virology 2015. “This is a tug-of-war.”

Scientists work back and forth with animals and other systems in laboratories. For example, Ellen Foxman, an immunologist at Yale University School of Medicine, is investigating viral interactions in laboratory-grown tissue made from authentic human airway cells. In one experiment, we studied swine flu and typical representatives of the rhinoviridae family.When researchers infected human tissues first with rhinovirus and then with swine flu, interferon Prevented the flu from gaining a footholdA similar study found that she also had rhinovirus infection. Hampered subsequent SARS-CoV-2 infection.

It’s hard to extrapolate people or populations from laboratory tissue, but Foxman believes the research reflects biological truth. It can be relatively tolerant,” she says. Foxman speculates that the protective effect probably lasts for days or weeks.

But don’t rely on the common cold to give you temporary immunity to other viruses. Interference is not guaranteed. It is certainly possible to detect multiple viruses at the same time. Also, interferons are not always beneficial. In some cases, it can not only make you more susceptible to infection, but it can also make you more susceptible to infection. A well-known example is that the flu makes people more susceptible to secondary bacterial infections.

It is still difficult to say how much role interference has played in stopping RSV and influenza in the global population in the ongoing pandemic. We believe that not enough people had Covid to disrupt other viruses on a large scale. (RSV We have reached an unusual summer peak In 2021, as people relaxed masking and other precautions.)

But by the second Covid winter in 2021-2022, Webby believes we’ll see evidence of interference at the population level. The flu had started to pick up in the fall, but then the Omicron variant of Covid popped up, he says. Influenza rates have declined even though people are returning to work and school and traveling on vacation. Covid-19 had a big advantage that season, he says. It doesn’t mean that Covid will always eclipse the flu in the future.

In the third Covid winter currently underway in the northern hemisphere, things are different again. Many people have acquired immunity to Covid due to recent bouts and vaccinations, but in recent memory, very few people have experienced RSV or influenza. The scene was set for a double comeback and an early and powerful blow.

Potential interference during the 2022-2023 triple epidemic winter will become more apparent once epidemiologists can look back at the season and see if each virus has taken turns. already, Plunge of RSV When The flu may have peakedCovid upwards after winter break. But there are still a few cold months ahead, providing ample opportunity for any of the trio to rise again.