We are in a major Covid wave. But how big?
The curves in some Covid graphs look quite steep again.
Reported levels of the virus in U.S. wastewater are higher than they have been since the first Omicron wave, according to data from the Center for Disease Control and Preventionalthough serious consequences still remain rarer than in previous pandemic winters.
“We’re seeing rates increase across the country,” said Amy Kirby, program leader for the CDC’s National Wastewater Surveillance System. The program now categorizes any state with available data on “high” or “very high” viral activity.
The wave could peak this week or soon after, modelers predict, with high levels of transmission expected for at least a month after that.
Hospitalizations and deaths have remained much lower than in previous years. There were approximately 35,000 hospital admissions reported in the last week of December – up from 44,000 a year earlier – and 1,600 weekly deaths in early December, down from 3,000. (At the same time, there were approximately 100,000 hospitalizations and 20,000 deaths each week in 2020.)
Many of the metrics used early in the pandemic have become far less useful indicators of the virus’s widespread spread, especially since federal officials abandoned expanded data-tracking efforts when they ended the emergency last spring in the field of public health. Higher population-wide immunity has led to fewer hospital admissions, even with high virus spread, and the sharp drop in Covid test results reported to authorities has made the number of cases much less relevant.
Wastewater testing remains one of the few reliable tools left to monitor the virus. It can signal the start of a surge before hospitalizations start to rise, and even includes people who don’t know they have Covid. For many who are still at higher risk from the virus — such as those who are older, have weakened immune systems or already have severe illness — it has become a crucial tool that helps them understand when to be especially careful.
But it’s an imperfect measure, most useful for determining whether virus spread is accelerating, not telling you exactly how many viruses are in circulation.
The data is often reported as normalized viral copies per milliliter or per gram, a figure that is virtually impossible to translate into precise case numbers, experts say. It’s also difficult to know how similar two different peaks are: a spike in the data may not mean exactly the same thing this year as last year.
That’s why many scientists studying the data will say only that it shows the country is in the middle of a big wave, not whether this winter’s wave is bigger than previous ones.
(The CDC doesn’t show actual concentration levels – the dashboard instead shows how much they’ve increased from when the spread was low. Above eight standard deviations is considered “very high.”)
Wastewater testing works at all because “everyone poops,” says David O’Connor, a virologist at the University of Wisconsin-Madison.
Samples of wastewater are collected at or en route to treatment plants and tested in a laboratory for viral RNA. But no two samples are perfectly comparable. The amount of RNA in the sample will fluctuate depending on many factorsincluding the local population at any given time – think an influx of vacationers to Miami or a college town that empties out in the summer – and how much other material, such as industrial waste, is in the system.
What experts really want to know, says Marisa Eisenberg, a professor at the University of Michigan who runs a wastewater monitoring lab for five sites, is how much virus there is relative to the number of people around — the wastewater equivalent of the population per capita. population. a number of cases.
Some labs “normalize” the data – that is, they adjust the denominator – by looking at the number of liters flowing through the plant, Professor Eisenberg said. But many sites use the so-called ‘pepper mild mottle virus’, a virus that infects pepper plants.
“People have studied this in human sewage and found that we release pretty consistent levels of this pepper virus,” she said. “So that’s a measure of how many people went to the toilet in the sewer shed today.”
Once Professor Eisenberg’s team normalizes the results, it will send data to the state and to the CDC, which collects information from sites through the whole country which together make up about 40 percent of the U.S. population.
The CDC then compiles its data and publishes state, regional and national data trends. (Two companies that analyze wastewater, Verily Life Sciences And Biobot analysisalso collects data from hundreds of sites and provides national and local images of the virus spread.)
But those national estimates can be tricky.
The sample population the CDC is looking at largely excludes people with septic tanks and cities without wastewater testing. Data breaches can happen, such as when the CDC changed contractor last year. Existing sites may stop testing and new sites may start up as the network changes and expands.
And while Biobot and Verily may use the same methodology and normalization across their sites, the CDC must determine trends based on data coming from different sites with a variety of methodologies.
Finally, there are changes to the virus itself that could make comparisons more difficult over time. Scientists monitoring these changes say there is evidence that this latest variant, JN.1, may multiply better in the intestines.
It’s still just a hypothesis, said Dr. O’Connor, the virologist. But it’s possible the virus is “a little more cozy in the gut” than it used to be, he said. If the hypothesis turns out to be correct, it could mean that infected people are shedding more viral copies than before. In the wastewater data, the same number of infections could look like much more Covid.
All of that adds up to significant uncertainty about how comparable the data is from year to year.
Michael Mina, epidemiologist and chief science officer at eMed, estimates that the true extent of Covid spread could be a lot higher or lower than this time last year. But there’s no doubt there are a lot of viruses out there, he said. And now much more than a few months ago.
Many experts studying this data recommend abandoning any idea of precision and just looking a little at the line’s recent trajectory. And if possible, look at your city’s wastewater, as data for a single location is typically more reliable over time than a national estimate.
“If you have vulnerable people in your community or family, you especially want to be aware when cases are increasing and take more precautions,” said Dr. Mina. “And if things slow down or come to a standstill, relax those precautions.”
Those precautions include wearing a high-quality mask, getting vaccinated, testing and staying home if someone is sick — and if someone at high risk is infected, taking Paxlovid.
Even in this new pandemic phase, people are still dying and can still become infected long Covid, says Maria Van Kerkhove, the technical lead on Covid for the World Health Organization. “While the Covid crisis is over, the threat is not,” she said.