Reviews |  This winter, it's the battle of viruses

Reviews | This winter, it’s the battle of viruses

Prior to the Covid-19 pandemic, little was done beyond flu shots to counter the seasonal respiratory viral infections that circulate in the general public in a typical year – such as influenza viruses, coronaviruses, rhinoviruses and more.

In the first two years of the pandemic, people largely got a reprieve from other viruses like influenza and respiratory syncytial virus, also known as RSV. Many people have gone two years without getting sick, perhaps a first in their lives and certainly unusual for children. The outbreaks that occurred were much smaller than usual.

Now those viruses are back. As winter approaches, pediatricians and hospitals report an influx of young RSV patients. Flu cases are skyrocketing. But why did these viruses disappear in the first place? And what does that mean for cold and flu season? Will there be a “triple epidemic” this winter?

Amid the holiday season, understanding the dynamics of virus rise and fall 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 prevailing virus in any given season, or even another new one for that matter. The knowledge gained from the Covid-19 pandemic could help to permanently reduce the number of respiratory viruses.

Scientists have ‌observed in past pandemics that a new virus can ‌‌affect the circulation of existing viruses. An example is the flu virus. During the last three influenza pandemics in 1957, 1968, and‌ 2009, influenza A viruses that were new to humans replaced some of the influenza viruses that were already circulating at the time, leading to the extinction of some older viruses.

‌Scientists don’t fully understand why this happens, but a few reasons are likely. On the one hand, when a serious new virus appears, people can change their behavior. It certainly happened earlier in the Covid-19 pandemic, when people started masking up, spending more time outdoors and limiting international travel. This has greatly ‌affected the spread of ‌respiratory viruses in general and may have reduced the number of susceptible people‌.

Second, when viruses are closely related, such as some influenza A viruses, there can be what is called cross-reactive immunity. This can happen when parts of a new virus are similar to viruses that are already circulating in the population. Previous immunity to an old virus can be greatly enhanced by infection with the new one. This can lead to stronger immunity against the old virus compared to the new one, and the old virus can disappear.

It is also thought that infection with one virus can stimulate a person’s innate, non-specific immune system – the body’s first line of defense against germs – which temporarily provides some protection against infection with another virus. For example, influenza made a brief return to the United States in December 2021, but was apparently overtaken by the emergence of the Omicron BA.1 wave at the start of winter. ‌Influenza cases spiked again when the Omicron wave subsided, and short-lived innate immunity could have played a role here.

So, given all of that, what’s going on right now with all of the disease?

‌The relative lack of virus circulation beyond SARS-CoV-2 for more than two years means population-level immunity is lower than it normally would be and people are more susceptible to the viruses. This is especially true for children, as many were born during the pandemic and have not experienced many viral infections. Adults who were exposed to circulating viruses before the pandemic may still be susceptible, as our immunity ‌declines over time.

Although it is difficult to predict exactly what will happen this winter, there will likely be many people who will fall ill with respiratory viruses. Children could get more illnesses over the next couple of years, before things settle down to a more steady pace.

This coronavirus has been dominant for so long and remains a serious threat, but baseline immunity against vaccination and infection has increased dramatically. Treatments are also available. So while SARS-CoV-2, the virus that causes Covid-19, now adds to the circulating viruses that people come into contact with, its mortality is expected to continue to decline over time (barring a more dangerous variant ). At present, it still kills around 300 people a day in the United States. But in the long run, it could become another circulating virus 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 start discussions about how to better control respiratory viral infections in general.

These viruses put considerable pressure on the health care system and the economy. Influenza viruses alone can cause up to approximately 50,000 deaths per year in the United States. Investing in measures such as improving ventilation‌‌ systems, especially in schools, could reduce the spread of many respiratory viruses. Wearing masks on public transport can protect you in times of high transmission. Strict adherence to staying home when sick – and policies allowing people to afford it – can also significantly reduce infections.

New vaccines that block infection and transmission of respiratory viruses are needed, but it may be years before they are available. In the meantime, get your flu shot and up-to-date Covid-19 shot, which provide significant protection against disease, especially serious illness. Now is the time to do it.

Florian Krammer is a professor of vaccinology in 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. His research focuses on how antibodies target and neutralize viruses and how to translate this knowledge into vaccines and therapeutics. Aubree Gordon is associate professor of epidemiology and director of the Michigan Center for Infectious Disease Threats at the University of Michigan. His work focuses on how infectious diseases like influenza and SARS-CoV-2 are transmitted and how people develop immunity against respiratory viruses.

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