Have anti-Covid measures killed seasonal flu?

In New York City, which publishes a handy daily tally of emergency room visits for flu-like illnesses and other conditions, the numbers are less than a third of normal for the first half of December.

This apparent absence of influence is almost certainly not just a fluke. Nor is it the result of a cover-up in which authorities count influenza cases as Covid-19 cases (apparently a popular theory in some denier circles). It is conceivable that the New York emergency room is less likely to count coronavirus cases as flu-like illnesses than last March, when Covid tests were scarce and healthcare workers less familiar with Covid symptoms. It's also possible that people are simply avoiding the emergency room if possible. But positive flu test results plotted on the US Centers for Disease Control and Prevention FluView show an even steeper decline.

That dark blue line that is virtually indistinguishable from zero for most of the year is 2020. The total of positive tests for the week ending December 5 – the most recent data available – was only 56. In the same week last year it was 6,435. The start of the flu season in the Northern Hemisphere is generally considered the 40th week of the year, which ended on October 3, making December 5 the end of week 49. Here is a decade of positive tests for the first 10 weeks of the flu season: yes, the 2011-2012 flu season started with even fewer positive tests. But this is misleading, given that the number of flu tests performed has increased a lot since then (although it's still far from the number of Covid-19 tests performed this year). The 511 positives in the first 10 weeks of the 2011-2012 flu season were on 40,150 tests, for a positive rate of 1.3%. This year's 602 positives are on 401,112 tests, for a positivity rate of 0.15%. Apparently, then, this flu season in the United States is on track to be significantly milder than what turned out to be the least worst of the last decade and possibly the least worst since the 1980s.

Similar trends are evident around the world. “In the temperate zone of the Northern Hemisphere, influenza activity has remained below inter-seasonal levels,” summarized the World Health Organization in its most recent weekly influenza update. That is, there is less incidence of influenza at this time, in late fall, than there is usually in the summer. WHO and other public health agencies are understandably reluctant to declare victory over the flu right now – it's still early in the season, and there's another disease out there that's wreaking havoc even as the flu is not. The CEO of smart thermometer company Kinsa Inc. is less reticent, telling the New York Times that "it looks like twindemic won't happen." And it's clear that since the flu seasons of 2020, which are already over, something out of the ordinary is happening.

Studies have found that in the United States and Europe the incidence of influenza declined much more dramatically than usual in late winter and early spring this year. And in Australia, where the peak of the flu season is generally from April to September and Covid-control efforts have been far more successful than in the United States or Europe, there has been no flu season.

Since early May (the last full month of autumn in the Southern Hemisphere), five flu tests have come back positive in Australia, according to the WHO's FluNet database. Since mid-July, no one has tested positive. Other countries in the temperate zones of the southern hemisphere have had similar experiences.

Why is all this happening? The push to get more people vaccinated against the flu this fall to avoid the dreaded twindemic may have had some impact, but that doesn't explain why the incidence of the flu plummeted last spring. The obvious explanation is simply that the things individuals and governments have done to slow the spread of Covid-19 have led to the spread of influenza, a respiratory disease that is similarly, if not identical, transmitted to a screeching arrest.

These measures have probably been more effective against the flu than against Covid, because the flu is much less contagious than Covid. A rough measure of contagiousness is the basic number of reproduction – the number of people that any person with the disease can be considered contagious if everyone is behaving normally. For seasonal influenza it is about 1.3, in influenza pandemics it was higher but still lower than 2. For Covid-19 it is probably between 2 and 4.

Wearing face masks, working from home, banning large gatherings and other social distancing measures – along with more people gaining immunity by contracting Covid-19 – appear to have brought the actual Covid breeding number in the United States to not much more than 1. Apparently, this also pushed the flu's effective reproduction number well below 1.

One lesson from this is that the often heard lament that the US and many European countries have failed in the fight against the pandemic is wrong. Of course, a quick glance at East Asia makes it clear that the West could have done much, much better. But given the success we have had in stopping the flu, it seems clear that we have also been successful in slowing down Covid. The resurgence of the disease in this fall was bad, but it could have been much, much worse.

Another lesson is that "non-pharmaceutical interventions," the art term for all the things we've done to slow the spread of Covid in anticipation of vaccines, should be a bigger part of the flu toolkit. . This doesn't mean we should close all borders and restaurants every winter, but low-cost measures like taking hand washing seriously, wearing a face mask when you feel unwell, working from home if you've been exposed, and keeping visitors. sick people and workers away from nursing homes could save thousands of lives every year. And if a new, deadly strain of pandemic flu arises like, for example, the 1918 variant (which was much more lethal than Covid-19, especially for young people), more expensive interventions would almost certainly be worth it.