The Times ponders a Covid mystery - why have surges faded after about two months?
Has Delta Peaked?
We investigate Covid’s mysterious two-month cycle
By David Leonhardt and Ashley Wu
Has the Delta-fueled Covid-19 surge in the U.S. finally peaked? The number of new daily U.S. cases has risen less over the past week than at any point since June, as you can see in this chart:
There is obviously no guarantee that the trend will continue. But there is one big reason to think that it may and that caseloads may even soon decline. Since the pandemic began, Covid has often followed a regular — if mysterious — cycle. In one country after another, the number of new cases has often surged for roughly two months before starting to fall. The Delta variant, despite its intense contagiousness, has followed this pattern.
Sadly, there is no particular explanation. And in any case, there is an obvious issue of aggregation - even if Florida and Texas have peaked that does not assure that New England, relatively calm so far, has also peaked.
They serve up some speculation:
Two possible stories
We have asked experts about these two-month cycles, and they acknowledged that they could not explain it. “We still are really in the cave ages in terms of understanding how viruses emerge, how they spread, how they start and stop, why they do what they do,” Michael Osterholm, an epidemiologist at the University of Minnesota, said.
But two broad categories of explanation seem plausible, the experts say.
One involves the virus itself. Rather than spreading until it has reached every last person, perhaps it spreads in waves that happen to follow a similar timeline. How so? Some people may be especially susceptible to a variant like Delta, and once many of them have been exposed to it, the virus starts to recede — until a new variant causes the cycle to begin again (or until a population approaches herd immunity).
The second plausible explanation involves human behavior. People don’t circulate randomly through the world. They live in social clusters, Jennifer Nuzzo, a Johns Hopkins epidemiologist, points out. Perhaps the virus needs about two months to circulate through a typically sized cluster, infecting the most susceptible — and a new wave starts when people break out of their clusters, such as during a holiday. Alternately, people may follow cycles of taking more and then fewer Covid precautions, depending on their level of concern.
Mr. Leonhardt had more at the end of July, including a "Stay Humble" message - since we don't know what's going on its hard to be sure how helpful our various interventions (other than vaccines!) have really been:
A more plausible explanation appears to be that Delta spreads very quickly at first and, for some unknown set of reasons, peters out long before a society has reached herd immunity. As Andy Slavitt, a former Covid adviser to President Biden, told me, “It seems to rip through really fast and infect the people it’s going to infect.” The most counterintuitive idea here is that an outbreak can fade even though many people remain vulnerable to Covid.
That’s not guaranteed to happen everywhere, and there probably will be more variants after Delta. Remember: Covid behaves in mysterious ways. But Americans should not assume that Delta is destined to cause months of rising caseloads. Nor should they assume that a sudden decline, if one starts this summer, fits a tidy narrative that attributes the turnaround to rising vaccination and mask wearing.
“These surges have little to do with what humans do,” Osterholm argues. “Only recently, with vaccines, have we begun to have a real impact.”
I don’t want anyone to think that Osterholm is making a nihilist argument. Human responses do make a difference: Masks and social distancing can slow the spread of the virus, and vaccination can end a pandemic.
The most important step has been the vaccination of many older people. As a result, total British deaths have risen only modestly this summer, while deaths and hospitalizations remain rarer in heavily vaccinated parts of the U.S. than in less vaccinated ones.
But Osterholm’s plea for humility does have policy implications. It argues for prioritizing vaccination over every other strategy. It also reminds us to avoid believing that we can always know which behaviors create risks.
Humility is in short supply these days.
Recent Comments