The Covid-19 delta variant is more contagious but it isn’t more severe

Jul 15, 2021 | 35 comments

By David Leonhardt

There are two basic questions to ask about any variant of the Covid-19 virus: Is it more contagious than earlier versions of the virus? And is it more severe?

A Covid-19 patient yesterday in St. Petersburg, Russia.

When a variant is more contagious, it leads to a rise in the number of infections, especially among the unvaccinated. When a variant is more severe, it causes worse symptoms for the average person who gets the virus and leads to a greater percentage of cases that result in hospitalization or death.

It is easy to confuse these two different concepts when a variant — like Delta — begins spreading. If the variant is more contagious, it often appears to be more severe as well because the increase in caseloads leads to an increase in the raw number of hospitalizations and deaths, as Dr. Robert Wachter of the University of California, San Francisco, explained to me.

In response, journalists and some experts talk about the new variant being “worse,” “riskier” or “more dangerous” — broad concepts that muddy the difference between contagiousness and severity. “Part of the problem is imprecision in language,” Dr. Rebecca Wurtz, an infectious-disease expert at the University of Minnesota, said.

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The difference between the two concepts is important. If a new variant is not actually more severe, it doesn’t present a greater threat to a typical person who contracts Covid. Vaccinated people would remain protected. For children too young to be vaccinated, serious Covid symptoms would still be exceedingly rare — rarer than many other everyday risks, like riding in a car — and still concentrated among children with other health problems.

After the Alpha variant began spreading late last year, many people assumed that it was both more contagious and more severe. The data soon told a different story, though: Alpha seems to be only more contagious.

Now the story may be repeating itself with Delta. It is significantly more contagious than even Alpha by almost every measure. It does not appear to be more severe, based on the data available so far.

That data remains messy, and it could change. You can find narrow statistics that point in any direction, variously suggesting that Delta is more severe, similarly severe or less severe than earlier versions of the virus. But the bulk of the evidence indicates no meaningful change.

“As far as anyone can tell, Delta isn’t more dangerous in the sense that it causes worse disease,” Wurtz told me. “It’s a sneaky opportunist, not a mayhem man.”

Janet Baseman, a University of Washington epidemiologist, said: “I have not seen compelling evidence that the Delta variant is more severe.” Dr. Paul Sax of Brigham and Women’s Hospital in Boston told me, “This sense of greater disease severity is more anecdotal than driven by actual data.” Dr. Eric Topol of Scripps Research said, “I don’t think it makes kids sicker.”

Dr. Aaron Richterman of the University of Pennsylvania said that he did not think Delta required vaccinated parents to behave differently than they did a few weeks ago. Richterman has young children himself, and his family has not changed its behavior, he said.

A good way to understand Delta is to look to England, where the variant has been circulating widely since May, longer than in the U.S.

If Delta were more severe than earlier versions of the virus, the percentage of cases leading to hospitalization or death should be rising. They’re not, as you can see in these two charts:

The average severity of Covid declined in the spring, thanks to England’s mass vaccination program. (The vaccines reduce severe Covid cases even more sharply than total cases.) Since the spring, severity has remained in the same tight range. The lines on those charts would probably have started rising in May or June if Delta were more severe.

In many ways, this picture should not be surprising. It is highly unusual for a virus variant to be both more contagious and more severe.

Delta, of course, is still a problem.

For unvaccinated older adults, Covid does not need to be additionally severe to be a mortal threat. The increased contagiousness of Delta has led to Covid surges across much of the globe, putting those unvaccinated adults at greater risk of contracting it.

As a result, vaccination has become even more important than it already was. In the U.S., regions with greater vaccination skepticism — which tend to be politically conservative areas — are now suffering larger outbreaks. In many other countries, where people have often not had the opportunity to be vaccinated, cases are also surging. Still, the global mass vaccination program is proceeding with agonizing slowness.

As has often been the case with Covid, the story is not a simple one. Delta is a menacing development in some places and may make little difference in others. “Delta is creating a huge amount of noise” in the U.S., Bill Hanage, an epidemiologist at Harvard, told The Times, “but I don’t think that it’s right to be ringing a huge alarm bell.”
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Credit: New York Times Morning Letter

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