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What happened to the flu - and what does it mean for the future of viruses that attack humans?

As a result of the restrictions such as social distancing, not only the spread of the corona but also that of the flu was limited. Will the absence of a flu epidemic in both hemispheres mean that we will not know which strain to prepare for next winter?

The corona virus versus the flu virus. Illustration: shutterstock
The corona virus versus the flu virus. Illustration: shutterstock

Something very strange happened this winter: cases of the flu decreased significantly, and along with them also other cases of milder 'colds'. At the same time, the number of patients with the new corona virus - SARS-CoV-2 jumped upwards. In the southern half of the globe, winter has already passed - and the flu has hardly affected the countries in that region. And on the other hand - and this is where things get really interesting - it turns out that certain viruses that cause minor respiratory diseases thrive and flourish.

What is going on here?

Contrary to the opinion of a number of conspiracy theorists, we do not confuse the disease Covid-19 with the flu and other respiratory diseases. It is possible to easily identify Covid-19 in patients using PCR and other tests, so it is very unlikely that there is confusion between the different diseases. So what's really going on?


What happened to the flu?

The accepted answer is that we used a variety of methods to fight the spread of the corona virus - wearing masks, maintaining social distance, washing and sanitizing hands, a dramatic decrease in the number of flights and, of course, closures. All of these resulted in a significant reduction in the ability of SARS-CoV-2 to spread, but they also had dramatic effects on less contagious viruses, such as the flu. And that's great - but in the long run of a year or two ahead, we may also see the negative results of this particular development.

Already in May, when the first wave of Covid-19 began to fade in some countries, it was clear that the flu season had ended prematurely in the northern hemisphere[1]. The initial thought was that simply fewer people came to the clinics and hospitals to check if they had contracted the flu, but this hypothesis was quickly ruled out. Although the number of people tested for the flu dropped by 61 percent, the rate of positive test results dropped by 98 percent![2] Even so, since the flu season started in the Northern Hemisphere back in 2019, before the lockdown measures were taken around March, enough people still got the flu at that time. Specifically, 38 million people in the United States. This is a low number compared to previous years, but not very unusual.

And what happened in the southern hemisphere, where the flu arrived at the same time as the period of closures and other preventive measures? There, almost no cases of influenza were detected in countries that took preventive measures against SARS-CoV-2. In fact, the number of flu cases dropped so dramatically that experts became suspicious. True, the flu is less contagious than SARS-CoV-2, but to that extent? And why was there also a significant decrease in flu cases in South American countries, where preventive measures against SARS-CoV-2 were not particularly successful?

The experts who studied the issue came to the conclusion that there are other factors that led to a decrease in flu cases. Although there is currently not enough data to determine unequivocally, some suspect that the lack of international flights has had a particular impact, since infectious patients are no longer crossing borders.

Another guess is that the increasing vaccination rate against the flu also contributed to the decrease in its incidence. In Australia, for example, the number of people getting vaccinated against the flu almost doubled in 2020. There is also an increase in the number of people getting vaccinated against the flu in the United States, and a little more than half of the adult population was vaccinated between 2019 and 2020.

On the one hand, this is a wonderful result. It means that fewer people will get the flu and accordingly the number of deaths from the disease will also decrease. We can also worry less about people getting the flu and SARS-CoV-2 at the same time, or even that flu patients will overwhelm hospitals and make it difficult for doctors to treat both diseases at the same time. And not only that, but as the flu becomes more difficult to pass from person to person, some of the rarer strains may decline further in prevalence. Maybe even forever.

On the other hand, it is very possible that the minority of flu cases will have unexpected consequences once the SARS-CoV-2 disappears from our lives.


The other viruses are also affected

Every year, virus researchers follow the development of the flu and predict which strain will reach us this coming winter. The fact that the flu barely raised its head last winter in the Northern Hemisphere will make it difficult for those prognosticators to prepare the adequate flu vaccine for 2021. What's worse: if people don't suffer from the flu for one or more winters, they may lose their natural resistance to it. If that happens, and if they don't get vaccinated against the new strains that will appear next winter, then we could suffer from a much more severe flu epidemic, once the restrictions designed to fight SARS-CoV-2 are eased.

But the flu is not the only virus affected by the new condition.

There are hundreds of different viruses that are responsible for the mild respiratory diseases we suffer from every winter. At the moment it appears that these viruses have also experienced a significant decline in their prevalence in the southern hemisphere. Thus, for example, the RSV virus, which causes five percent of deaths in children under the age of five worldwide, declined in its prevalence in Australia by 98 percent in the winter of 2020[3]. But this virus is also expected to return once the restrictions are lifted, and some experts suspect that since many children will go through this winter without getting RSV, their immune systems will not develop resistance against it. And since there is no vaccine against the virus, we may see a large increase in RSV cases once the kids all go back to school.

But not all viruses have decided to take a break in the past year. The only virus that actually got stronger and stronger is the rhinovirus, which causes many cases of the common cold. There are more than a hundred different strains of the virus, and although there was a slight decrease in its prevalence in the summer in England, the prevalence jumped back up as soon as schools opened. The number of rhinovirus cases also spiked in winter in the southern hemisphere.

Why? How is the rhinovirus different from all the others?

The unpleasant answer is that no one is safe. One of the thoughts is that since the rhinovirus, unlike the corona viruses and the flu, does not have a fatty envelope around it, which can be damaged by various soaps. The rhinovirus may also be able to stay longer on surfaces and therefore can pass between hands, tables and doorknobs.

And who knows - everything can still change.


The virus ecosystem is being disrupted

Over the past hundreds of years, a complex ecosystem has developed in which viruses fight with each other for the right to infect us first. Because many viruses stimulate the immune system to act generally, they prevent other viruses from successfully spreading in the infected body. In other words, rhinoviruses fight the flu, the common cold-causing corona viruses, and hundreds of other viruses. And each of them in turn struggles with everything else. So, for example, there is evidence that rhinoviruses neutralized the flu epidemic that occurred in 2009[4].

Now suddenly, this whole ecosystem has been shaken. The prevalence of hundreds of viruses has dropped dramatically, while some other viruses such as the rhinovirus are able to take advantage of the new niche that has opened for them among all those people who have not been infected with the other viruses. This may be a negative development... or a positive one. It is even possible that the rhinoviruses could prevent some cases of infection with SARS-CoV-2! As in any case where an ecosystem is undermined, it is difficult to know what the consequences will be.

Either way, according to the article in Nature (from which I took much of the data that appeared in this post), most researchers agree that it's worth preparing to be safe for the more pessimistic post-corona scenarios[5]. We may experience a particularly severe flu season. Perhaps widespread outbreaks of RSV. Maybe not - but you should still prepare, and not think that as soon as the corona disappears, the world will go back to normal. For better or worse, the world has undergone a reboot, and now it seems the virus ecosystem has experienced a similar reboot.

Good luck to all of us, and go get vaccinated. From corona, flu and everything else. Going to be interesting in the next few years.


[1] https://www.nature.com/articles/d41586-020-01538-8

[2] https://pubmed.ncbi.nlm.nih.gov/32941415/

[3] https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciaa1475/5912591

[4] https://pubmed.ncbi.nlm.nih.gov/33103132/

[5] https://www.nature.com/articles/d41586-020-03519-3

Dr.Roey Tsezana is a futurist, lecturer and author of the books "The Guide to the Future" and "Those Who Control the Future"

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