Flu season 2020-2021: it didn’t happen. Can we repeat it next season?

The sum of all masks, distance, closing deals and other precautions, although imperfect, were not enough to stop the Covid-19 pandemic on its way. But there is a silver lining: has it was enough to virtually eliminate the flu this season.

The Center for Disease Control and Prevention (CDC) says that, as of January 30, there have been only 1,316 positive cases of flu in its clinical surveillance network since September. Around that time, last year, it registered 129,997 positive cases of flu in the same period.

Part of the drop may be because people are not going to be tested for the flu or are at home fearing that their symptoms are a Covid-19 infection. But the researchers think the decline in real cases is real and sharp.

It is not just confirmed cases that are inactive. The CDC’s syndromic surveillance system – which tries to track the disease based on people who come to clinics with symptoms – is experiencing historically low levels of flu.

Tim Ryan Williams / Vox

In the last flu season, the CDC estimated that the virus was responsible for “38 million illnesses, 18 million medical visits, 405,000 hospitalizations and 22,000 deaths”.

The flu trends this year mean that “we have found a way to potentially decrease tens of thousands of deaths each year,” said Seema Lakdawala, a flu researcher at the University of Pittsburgh.

This news contains a silver lining and a warning.

The good news is that we have seen how effective our collective behavior can be in reducing the burden of flu. And because of that experience, we can be better prepared to stop a future flu pandemic in its path.

The warning: as people are not getting the flu this year, it means that more people will be susceptible to catching it next year. Children are especially vulnerable to catching the flu, and next year, there will be more children than ever who have never caught the flu in their lives. This will also make them more likely to spread. Some adults may also be more vulnerable to getting flu in the next year. The immune memory of the virus fades over time, and when the next flu season begins, it will be a year since many adults have not received the flu vaccine for the last time.

To lessen the overall burden of future flu seasons, we will need to remember a few things we learned during the pandemic. It will not be sustainable to maintain all the precautions of Covid-19 after the pandemic has subsided. But we must spend some time thinking carefully about the changes that must remain.

The flu season that was not

This flu season, more or less, didn’t really happen. It is not entirely a surprise: during the southern hemisphere flu season last year (which occurs during the northern summer – from May to September), cases and deaths also fell.

That said, the researchers did not assume that the flu would not spread this winter.

“Until a few weeks ago, I didn’t feel like we were out of danger yet,” said Shweta Bansal, an epidemiologist and biologist at Georgetown University. But now that it is February – usually when the flu season peaks in the United States – she feels that a flu wave is just not coming. And it’s not just the flu: other respiratory viruses, with the exception of rhinoviruses, have also fallen dramatically.

This is a great relief: at the beginning of the pandemic, scientists were very concerned about the pressure on our health systems if the flu circulated on a large scale alongside Covid-19. The quiet flu season means that more hospital beds and equipment are available for Covid-19 patients, and the healthcare professionals who care for these patients are less stressed than during a year of normal flu.

The researchers do not believe that the sharp drops are due to more people getting flu shots this year. “Vaccination is helping, but there is absolutely no way that increasing vaccination on its own is responsible for it,” said Stephen Kissler, a Harvard epidemiologist. Yes, the uptake of the flu vaccine has increased (about 15.5 million more doses), but it is not enough to explain what happened to the flu. (Flu is also most commonly transmitted from contaminated surfaces. Therefore, routine surface cleanings can be a theater of hygiene when it comes to Covid-19, but it can help when it comes to flu.)

What is responsible? “As far as I can tell, the most consistent explanation is simply that wearing masks and distance really did it,” says Kissler.

Why the flu was defeated, but not Covid-19

How could wearing a mask and detachment stop the flu, while Covid-19 remains furious?

This goes back to something that scientists were saying at the beginning of the pandemic: Covid-19 is much, much worse than seasonal flu because it is more contagious.

Scientists describe the contagiousness of a disease with a number called R0 (pronounced r-nothing). The number describes, on average, how many new cases each case of disease starts to generate. For seasonal flu, the R0 is between 1 and 2. For Covid-19, it is more likely between 2 and 3, if not a little higher.

Our collective actions have reduced Covid-19’s effective R number to just over 1. As long as the R number is greater than 1, the virus will continue to spread. But, when it comes to the flu, all that collective action did bring the effective R for the flu below 1.

For Kissler, this is textbook epidemiology: “Anything less contagious [than Covid], but which spreads in a similar way, will be brought well below the R limit of 1, and will simply be eliminated. “

We can feel good because our efforts have eliminated the flu. It is one less burden to deal with an already difficult year. But there are also some optimistic conclusions for the future. Let’s say there is another pandemic respiratory virus, but a little less contagious than Covid-19. If we brought in the same collective action force to contain the new virus as now, this pandemic would probably be stopped.

We will learn more about flu transmission as society returns to normal

Scientists do not know, precisely, which public health measures Covid-19 are contributing most to the drop in flu cases. But they are suspicious. “It is the lack of travel, the closing of schools and the distance and masking that are making the biggest difference,” says Helen Chu, a doctor and epidemiologist at the University of Washington who studies flu.

It is difficult to say with certainty what contributed most. “Flu makes children sick or takes them to the hospital disproportionately, and things like closing schools have a greater effect on flu than on SARS-CoV-2,” says Chu.

But then, “I can’t imagine closing schools doing most of that, because so many places have managed to keep schools open and still don’t have the flu: Australia, for example. They still kept schools open and had no flu ”.

Although scientists currently do not know how much each intervention is contributing to the fall of the flu this year, they see opportunities to learn more.

“I hope someone will analyze this in the United States and try to find out by state, because there are so many regional differences in understanding social masking and distance and closing schools,” says Chu. Perhaps these differences in policy and behavior can be correlated with cold or flu results to better understand what worked.

One thing that will help even more: scientists will watch what happens when Covid-19 precautions are lifted. “We will begin to see the circulation of influenza and other respiratory viruses,” says Kissler.

Thus, scientists can potentially see what restrictions and changes in behavior lead to a greater resurgence of respiratory viruses.

We can reduce the burden of seasonal flu – without getting stuck

We can use the pandemic to think about how society can change to prevent further infections and deaths in the future. “Seasonal influenza viruses are still a major burden on public health,” says Lakdawala. This will still be true after the end of the pandemic, and we must remember some lessons from the pandemic to deal with it.

For example, in the past, if I had been exposed to someone with the flu, or maybe just a cold, I wouldn’t have thought of any of that. In the future, after the pandemic, I would consider not meeting with friends or going to the office after an exposure (flu has a shorter incubation time than Covid-19, so you wouldn’t need to be quarantined for that long, probably just up to four days). And I would wear a mask in public when I was sick or after being exposed to the virus. The flu, like Covid-19, can spread in the absence of symptoms and people can infect others before they feel sick.

We may even want to be more cautious during the flu season and wear masks around other people, regardless. “Wearing masks in the winter, I think it’s something that may have come to stay,” says Kissler. In general, there should be greater acceptance of wearing masks in public, so it is less likely to attract glances or confusion.

In the future, employers and employees may have a clearer understanding of the importance of staying home and obtaining sick leave when they become ill with the virus.

“We have a lot of studies that asked people, ‘Did you stay home because you had the flu? And they said, ‘No,’ ”says Chu. “We think that coming to work is essential. And we think that staying at home when you’re sick is a sign of weakness. I think that will change completely. ”In addition, parents who care for a child with the flu must have time and resources to stay at home, so that they do not spread it to others.

Schools can also use remote learning more carefully. If a district sees an outbreak of flu in their schools, they can temporarily switch to remote education to prevent a major outbreak in the community.

All of these actions must go beyond ensuring that as many people as possible receive the flu vaccine. Injections are usually only 40 to 60 percent effective in preventing someone from developing flu symptoms, so even though everyone gets vaccinated every year (and usually only about half of Americans do), we should still record the new lessons that we learn.

We need to be vigilant for the next flu season

We don’t need blockages to mitigate future flu seasons. They would be very heavy – and needlessly excessive – in a typical flu season.

Still, we will need a good deal of surveillance, especially in the coming flu season. Scientists are not sure how the flu will evolve in response to this strange new environment.

“We have no idea how the elimination of influenza for an entire year affects its evolution,” says Kissler. This may make it more difficult for vaccine developers to choose the right strains for next year’s vaccines. “We don’t know if it will be easier to predict next year’s flu strain, because it is not spreading so much. Or if it will be much more difficult, because it went through this very tight bottleneck that we call an evolutionary bottleneck. “

Here’s what we know: we can fight the flu with our behavior. Covid-19, in part, “showed us how to do it,” says Kissler.