How COVID-19 ended the flu season before it started

In the third week of 2021, clinical laboratories across the country tested 23,549 samples for influenza. Of these, only 0.3 percent (65 tests) tested positive – a number that is, to say the least, absolutely wild.

“Typically, at this time of year, we would have positive results of 20 to 30%,” said Lynnette Brammer, head of the Domestic Influenza Surveillance team at the Center for Disease Control and Prevention.

Although the United States continues to fight COVID-19, it has apparently managed to get the flu. Since the end of September, the combined total of positive cases of influenza identified by public health and clinical laboratories is less than 1,500. There are high schools with more people. The phenomenon doesn’t just occur in the United States – worldwide, flu rates are almost low. When you align several years on the same chart, it may even seem like there are no flu cases this year. That is how we are out of step with the norm.

These data are mind-boggling, especially since many observers spent the autumn worried about a “twindemia”. The tiny flu season also raises some questions: how can there be so many cases of one respiratory disease and so few of another? Why is there not always an organized correspondence between places with less COVID-19 and places with less flu or vice versa? And seriously, it’s the flu numbers really so low?

The answers are complex, although this tiny flu season is a simple fact.

We don’t track the flu in the same way that we track COVID-19. The average seasonal flu exists in a strange liminal space, serious enough to keep an eye on, but also not so serious that we are literally trying to count each case. After all, most people who get the flu don’t even bother to get tested. They will have some bad days in bed (if they are able to take time off work) and, otherwise, they will live basically without change.

Instead, influenza case counts come from a few different surveillance systems, including a network of about 100 public health and 300 clinical laboratories participating in virological surveillance, reporting numbers of weekly tests and positive cases to the CDC. There is also a network of doctor’s offices reporting cases of “influenza-like illnesses”, a network of hospitals reporting laboratory confirmed cases and mortality surveillance data from the National Center for Health Statistics, which obtains numbers for flu, pneumonia and other respiratory diseases directly from death certificates. Apart from the NCHS numbers, none of these networks represent truly national numbers. They are a sample, from all over the country, and are reported voluntarily. That is why the annual numbers of flu cases (and even deaths) are an estimate, extrapolated from the narrowest picture we can see.

All of this to say that there are almost certainly more than 65 cases of influenza across the United States a few weeks ago. This number represents what was counted in the member laboratories – not what was in total. And even that number is probably underestimated, experts said, because people with symptoms of respiratory disease are now significantly more concerned about COVID-19 than flu. “If people are not being tested, we can’t find them,” said Janet Hamilton, executive director of the Council of Territorial and State Epidemiologists. “And the protocol is not to test for flu if you are negative for COVID.” There are probably many people whose fears of COVID-19 infection are assuaged by a negative test result and who go home and deal with whatever illness they have. Does without seeking any medical treatment.

That said, Hamilton and others have told me that there is reason to believe that real cases of influenza are, in fact, decreasing. The fact that we are witnessing insignificant flu seasons in other countries, which tell their flu cases differently from ours, is part of the reason. In addition, hospitalizations for influenza in the United States have also decreased. “People in the hospital are properly diagnosed. So it’s a better indication of low circulation in people, ”said Adolfo García-Sastre, director of the Influenza Pathogenesis Research Center at the Icahn School of Medicine in New York. Between October 1 and January 23, there were a total of 142 hospitalizations for flu confirmed in the laboratory in this country – 0.5 per 100,000 Americans in almost four months. Typically, at this time of year, we’re talking about tens per 100,000 every week.

This massive change, experts said, is probably linked to the precautions we take to avoid catching COVID-19: wearing a mask, social detachment, obsessive surface cleaning (which does not help much to prevent COVID-19, but is probably preventing flu) and even keeping children out of the classroom. “The main vector of the flu is children,” said David Topham, co-director of the New York Influenza Center of Excellence in Rochester. If they can’t breathe normally, they can’t transmit that much flu either. And that trick still works, even if the flu isn’t the reason to keep them at bay.

The flu has not been our target with all these interventions, but we have certainly had a good beating. That’s because flu is not as transmissible as COVID-19. R0 (pronounced R naught) – the number that quantifies the average number of people who will catch the virus from a single infected person – is significantly lower for influenza than for COVID-19. “R zero is usually around 1 or 1.5 for the flu. And for SARS-CoV-2 it is between 2 and 4 ”, said Topham, referring to the scientific title of the new coronavirus. “When you do masking and social distance, you can artificially decrease R zero. This probably pushes the flu to a range below 1. Someone is infected, infects less than someone else, and then it is not viable. “

Our strategies are working on COVID-19 as well. Not so drastically, because it was more likely to spread to more people to start with.

This does not fully explain why places that took masking and detachment very seriously – Australia, for example – and places that do not – the US, say – are experiencing an almost nonexistent flu season. The significant reduction in international travel probably played a role in this, said Brammer. Normally, our flu season follows that of the southern hemisphere. But if there weren’t too many there, and there weren’t too many trips to carry the virus – flu can’t travel.

Meanwhile, in the United States, flu rates have remained low both in states that have instituted measures such as masking mandates and in those that have not. And García-Sastre said that this is probably due to a small concept that you may have heard of: herd immunity. Just as you don’t need to vaccinate absolutely everyone to get a benefit in reducing disease transmission, you probably don’t need to have the entire population wearing masks and practicing social detachment to see a benefit.

Still, scientists aren’t sure what’s going on because the problem with a very, very tiny flu season is that it doesn’t leave enough cases to make solid statistical inferences. We don’t know, for example, much about what happens when you get the flu and COVID-19, because there aren’t enough cases to do good research. We don’t really know how this bottleneck is affecting which strains of flu are circulating for the same reason. We don’t even know for sure if it is the masks and the distance that are crushing the flu, because there are so few cases of flu left to be examined.

And that is an irony that could end up affecting future flu seasons. Get the flu shot. Experts use data from previous flu seasons to decide which strains people should be vaccinated against. “It will certainly make the selection of viruses for the vaccine in the coming years more challenging. We just don’t have a lot to look at all over the world, ”said Brammer. “What will happen in the long run, I don’t know.”