Bay Area sewage surveys suggest COVID surge could be largest yet

Surveys of coronavirus levels in Bay Area sewage suggest the region’s relentless spring surge of COVID likely rivals omicron’s winter wave in terms of the number of people currently infected — in fact, this push may be the largest yet in some places. But capturing the scale of the disease and letting the public know the relative risk of getting sick is becoming increasingly tricky, health experts say.

The huge winter surge peaked in the Bay Area with more than 20,000 COVID cases reported daily in mid-January. Currently, the region is seeing far fewer cases – around 3,500 a day this week – but experts believe the true number of infections could be 2 to 10 times higher as many more people use home tests, whose results are not reported to county or state public health authorities.

In all previous outbreaks, the level of coronavirus detected in sewage closely matched the number of cases when plotted on a curve. During the omicron winter, for example, virus levels in sewage and the number of reported cases began to rise sharply in mid-December, peaked over a period of a few days, and then fell rapidly.

This spring surge broke that pattern – for the first time, case counts and viral levels diverged. In the past two months in particular, viral levels in sewage have risen again, while reported cases have increased at a much slower rate.

The two measures and their disparate portraits of what’s happening in the Bay Area underscore the challenges of tracking COVID as the region goes through its third year of the pandemic, health experts say. People need to consider a range of data points — from sewage monitoring and case counts to hospitalizations and even anecdotal reports among friends — to understand their personal risk of becoming infected and decide what precautions to take.

“Sewage surveillance indicates that there is a lot more disease than what we see in the number of cases,” said Dr. George Rutherford, an infectious disease expert at UCSF. “People should interpret that as there is a risk there, and if they go into crowded indoor environments or even very crowded outdoor environments – I’m thinking of the Stern Grove (Festival) – they should be careful. Pick and choose what you want to do. Use your mask.

Studying the amount of virus found in sewage began early in the pandemic in some locations and has become an increasingly popular surveillance tool in the United States over the past two years. It’s still not a perfect measure of how far the disease has spread in a community, and scientists have yet to come up with a formula to consistently translate sewage viral levels into infection rates. infection in the community.

But as other measures — and especially reported cases — become less reliable, sewage monitoring is appealing to many experts as a relatively stable and unbiased marker of COVID transmission.

“This push really highlights the utility of wastewater,” said Stanford wastewater monitoring expert Alexandria Boehm, who has conducted wastewater surveys in the Bay Area. “If you look at the data, the current surge is of the same order of magnitude as omicron’s first surge in December and January.”

Scientists are developing the wastewater metric by studying the concentration of viral RNA in wastewater. Boehm’s team at Stanford, for example, is analyzing sewage samples from a dozen locations in and around the Bay Area for several key mutations, including two that are found in all coronavirus variants currently in development. traffic.

Sewage monitoring is considered more reliable than other means of tracking COVID, in part because there is no human interference – some amount of virus is excreted when someone becomes infected, and Everyone’s poop ends up in the sewage. Tracking the virus in sewage does not depend on whether people decide to get tested, or where they do it.

Other metrics, like COVID hospitalizations and the rate of positive tests, can also be used to gauge the magnitude of a flare-up, but they also have shortcomings. Hospitalizations are up to a week or two behind cases, so they’re not ideal for understanding what’s going on right now. And they paint an incomplete picture of the true number of infections in the community, especially now that fewer and fewer COVID patients are ending up in hospital thanks to vaccines. The positive test rate is a little more reliable, but it can also be influenced by human bias – people are more likely to get tested if they have symptoms, to start with, or if they already have a positive result during a home test.

Currently, hospitalizations and the rate of positive tests show the Bay Area is in the midst of a significant surge, but not as large as the winter surge.

It is unclear whether this surge has surpassed the omicron wave in terms of daily infections. Rutherford assumed cases were now about 60% of omicron’s peak, “but maybe that’s just me putting on my happy face,” he said. Health officials in Santa Clara and Alameda counties said they don’t believe this surge has quite reached omicron levels — but added they couldn’t know either. for sure.

“The sewage tells us that we are still stable at very high levels of COVID transmission – very similar to what we were in January earlier this year,” said Michael Balliet, deputy director of public health for the county of Santa Clara. “I don’t think we’re quite on the same level, but we’re certainly pretty close.

“It’s important that people continue to practice all strategies — testing, wearing face coverings, getting vaccinated and boosting if you haven’t already — to reduce their risk,” he said.

Dr. Nicholas Moss, the Alameda County health officer, said he uses multiple metrics to get a sense of what’s going on in his community, but he appreciates that many people don’t have the time. or the interest in paying such close attention. This is becoming increasingly true as the pandemic drags on and people have other priorities in their lives to distract them from COVID. With that in mind, he said simply relying on social media to keep tabs on the virus might be the best way for many people to assess their own risk.

“Honestly, I find in some ways the easiest way for people to get an idea of ​​the amount of COVID is just: do you hear of a lot of people getting COVID? It’s a good sign that for you there’s a lot of COVID out there, there’s a risk and you need to be careful,” Moss said. “It’s very unscientific, but people really relate to it. If the person across the way has COVID and my aunt has COVID and we heard about a cluster of cases at the local daycare – that kind of signal, people should pay attention.

Erin Allday is a staff writer for the San Francisco Chronicle. Email: [email protected]: @erinallday

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