Before the Covid-19 pandemic, little was done beyond flu shots to counter the seasonal respiratory virus infections that circulate among the general public in a typical year — such as influenza viruses, coronaviruses, rhinoviruses and more.
During the first two years of the pandemic, people largely got a reprieve from other viruses like flu and respiratory syncytial virus, also known as RSV. Many people went two years without getting sick, perhaps a first in their lifetime and certainly out of the ordinary for children. Outbreaks that did occur were much smaller than usual.
Now, these viruses are back. Heading into winter, pediatricians and hospitals are reporting an influx of young patients with RSV. Flu cases are spiking. But why did these viruses disappear in the first place? And what does this mean for the cold and flu season? Will there be a “tripledemic” this winter?
Amid the holiday season, understanding the dynamics of how viruses surge and plunge helps explain why so many people, especially young children, are sick right now — or will be this winter. But as a society, we don’t have to be at the mercy of the dominant virus in a given season, or even another new one for that matter. The knowledge gained from the Covid-19 pandemic could help lower the toll of respiratory viruses for good.
Scientists have observed in past pandemics that a new virus can affect the circulation of existing ones. One example is the influenza virus. During the last three influenza pandemics in 1957, 1968 and 2009, influenza A viruses that were new to humans replaced some of the flu viruses that were already circulating at the time — resulting in the extinction of some of the older viruses.
Scientists do not fully understand why this happens, but a few reasons are likely. For one, when a new and serious virus emerges, people may change their behavior. This certainly happened earlier in the Covid-19 pandemic, when people began masking, spending more time outside and limiting international travel. This greatly affected the spread of respiratory viruses generally, and may have reduced the number of susceptible people.
Second, when viruses are closely related, such as some influenza A viruses, there may be what’s called cross-reactive immunity. This can occur when parts of a new virus are similar to viruses already circulating in the population. Prior immunity against an old virus may be substantially boosted by infection with the new one. This may result in stronger immunity to the older virus compared to the new one, and the older virus may peter out.
It’s also thought that infection with one virus might rev up a person’s innate, unspecific immune system — the body’s first line of defense against germs — which temporarily provides some protection against infection from another virus. For example, the flu made a short comeback in the United States in December 2021, but it was seemingly outcompeted by the emergence of the Omicron BA.1 wave in the early winter. Flu cases rose again when the Omicron wave subsided and short-lived innate immunity could have played a role here.
So given all of this, what’s happening right now with all the sickness?
The relative lack of virus circulation beyond SARS-CoV-2 for over two years means immunity at the population level is lower than it normally would be, and people are more susceptible to viruses. This is especially true for children, as many were born during the pandemic and did not experience many viral infections. Adults who have been exposed to circulating viruses prepandemic can still be susceptible, as our immunity wanes over time.
Though it’s hard to predict exactly what will happen this winter, there will likely be lots of people getting sick with respiratory viruses. Kids might come down with more illnesses for the next one to two years, before things settle into a more regular rhythm.
This coronavirus has been dominant for so long, and remains a serious threat, but baseline immunity from vaccination and infection has greatly increased. Treatments are also available. So while SARS-CoV-2, the virus that causes Covid-19, is now added to circulating viruses people come into contact with, its deadliness should continue to decline over time (barring a more dangerous variant). Right now, it still kills about 300 people a day in the United States. But in the long run, it could become yet another virus circulating in a given season, sometimes losing out in terms of infections to the competition.
The world has learned so much about viruses and immunity during this pandemic and it’s time to engage in discussions about how to better control respiratory virus infections in general.
These viruses cause substantial strain on the health care system and the economy. Flu viruses alone can cause up to around 50,000 deaths per year in the United States. Investing in measures like improving ventilation systems, especially in schools, could lower the spread of many respiratory viruses. Wearing masks on public transportation can protect you in periods of high transmission. Strict adherence to staying at home when sick — and policies so people can afford to do so — can also significantly reduce infections.
New vaccines that block infections and transmission of respiratory viruses are needed, but it may take years for them to become available. In the meantime, get your flu shots and updated Covid-19 booster vaccinations, which provide significant protection against disease, especially severe disease. The time to do so is right now.
Florian Krammer is a professor in vaccinology at the Department of Microbiology at the Icahn School of Medicine at Mount Sinai and co-director of the Center for Vaccine Research and Pandemic Preparedness. Aubree Gordon is an associate professor of epidemiology and the director of the Michigan Center for Infectious Disease Threats at the University of Michigan. This article originally appeared in The New York Times.