Bay Area wastewater surveys suggest COVID surge could be biggest yet

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

The huge winter surge peaked in the Bay Area at more than 20,000 reported COVID cases a day in mid-January. Currently, the region is recording far fewer cases — roughly 3,500 a day as of this week — but experts believe the actual number of infections may be anywhere from 2 to 10 times higher, as so many more people use home tests, the results of which are not reported to county or state public health authorities.

In all previous surges, the level of coronavirus detected in wastewater closely matched case counts when plotted on a curve. In the omicron winter, for example, both wastewater virus levels and reported case counts started to spike sharply in mid-December, peaked over a period of a few days, then quickly plummeted.

This spring surge broke that pattern — for the first time, case counts and viral levels have diverged. Over the past two months in particular, viral levels in the wastewater shot up again, while reported cases have climbed at a much slower pace.

The two metrics and their disparate portraits of what’s happening in the Bay Area underscore the challenges of tracking COVID as the region lurches through its third year of the pandemic, health experts say. People must take into account an array of data points — from wastewater surveillance 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.

“Wastewater surveillance says there’s lots more disease out there than what we see in the case counts,” said Dr. George Rutherford, an infectious disease expert at UCSF. “People should be interpreting this as there is risk out there, and if they enter crowded indoor settings or even very crowded outdoor settings — I’m thinking of the Stern Grove (Festival) — they need to be careful. Pick and choose what you want to do. Use your mask.”

Studying the amount of virus found in wastewater began early in the pandemic in some places, and has become an increasingly widespread surveillance tool across the United States over the past two years. It’s still not a perfect measure of how much the disease is spreading in a community, and scientists have not yet worked out a formula for consistently translating wastewater viral levels to community infection rates.

But as other metrics — and especially reported cases — become less reliable, wastewater surveillance is winning over many experts as a relatively stable, unbiased marker of COVID transmission.

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