Why Do Covid Surges Seem To End As Soon As They Begin?

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The omicron variant of Covid-19 was discovered less than three months ago, but it rocketed case numbers to record highs. Yet almost as rapidly as they rose, new infections plummeted in countries like the United Kingdom, South Africa, and now the United States, reports Vox.

Why omicron shot up — and down — so fast

The omicron variant of SARS-CoV-2, the virus that causes Covid-19, appeared at just the right time to cause a huge infection spike. It took root in the Northern Hemisphere as holiday travel picked up and cooler temperatures pushed people indoors, helping it travel long distances and spread locally through person-to-person contact.

Omicron also had the right mix of traits to catch fire. The omicron variant contains mutations that allow it to better evade immune protection while spreading faster than any prior known variant. Even people vaccinated against Covid-19 began getting infected in large numbers as protection from their initial doses started to waver, though most experienced mild symptoms. All these factors together led to lots of infections happening very quickly.

While omicron has been the most extreme example of this phenomenon, earlier variants also caused sharp spikes and declines.Most of these peculiar “stalagmites” in South Africa were symmetrical except for the delta wave last summer, which saw a brief resurgence on its way down.

A key variable is the basic reproductive number of the virus, or R0, which is the average number of people that one infected individual tends to infect.

As more people get infected with a coronavirus variant, there are fewer people left to infect. When the basic reproductive number falls below one, new infections reach their peak and then decline. To plateau, the rate of new infections has to stabilize somewhere near one, but that would require an unusual set of conditions, according to Lessler.

Farr’s law

The idea that disease outbreaks are generally symmetrical is an old one. William Farr observed in the 1840s that smallpox epidemics followed a mathematical pattern, though his formula, known as Farr’s law, resulted in a bell-shaped curve. But diseases rarely follow such neat curves.

“That has been generally discredited as a ‘law’ since it doesn’t allow for things like changes in susceptibility due to different levels of immunity/immune waning, movement in and out of populations, and changes to risk and exposure behaviors,” said Murray.

That’s been evident during the Covid-19 pandemic. Some countries like South Korea saw more gentle hills as different variants took root, while others like Brazil experienced asymmetrical, jagged peaks throughout the pandemic. Some of that is due to delays in identifying and reporting cases.

In some places, variants like delta and omicron overlapped. At the country level, case curves can change shape as the pandemic spreads over time from urban to rural areas or can peak at different times depending on the region.

So both the shape and the size of an infection spike can be altered with public health tactics. Over time, as immunity builds up in the population, experts expect to stop seeing tall, sharp spikes in Covid-19 cases. The virus probably won’t go away entirely, but case counts could form seasonal waves as new variants arise, immunity wanes, and exposure opportunities increase, according to Mokdad.

Omicron cases are poised to drop further

Covid-19 cases spurred by omicron appear to have peaked already in the US, but the health care system is still facing a stressful time ahead.
If public health measures like masking and social distancing are relaxed too soon, cases can bounce back up on their way down. The UK, for instance, reopened schools and relaxed Covid-19 rules before the omicron wave flattened out. Then infections stopped dropping.

The virus is always changing: Omicron now has a subvariant called BA.2 that is gaining some ground, though it’s not yet clear what it means for the pandemic overall.

The more the virus spreads, the more likely it is to mutate in dangerous ways. As the current variants have shown, they can quickly spread around the world, regardless of where they originate.

The recurring spikes of Covid-19 cases, fueled in part by variants, should inspire us to redouble our efforts at controlling the disease, especially with vaccines. “We’re still struggling to avoid these peaks as vigilant infection prevention efforts and global vaccine equity have been a challenge,” said Popescu.

A more robust global vaccination effort, coupled with better disease surveillance to catch variants before they spell trouble, could prevent the next wave and finally start to bring the pandemic under control.

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Source: Vox  

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