As dramatically as coronavirus numbers spiked across the Bay Area, California and U.S. during the winter surge, those numbers have plummeted in the past month.
Nationwide, new cases are down more than 70% and hospitalizations down nearly 50% since mid-January. In California — an epicenter of the deadly winter surge — new cases have declined 83% since peaking this year on Jan. 14. Hospitalizations are below 10,000, less than half of what they were at the beginning of the year.
In the Bay Area, just over 9,000 new cases were reported during the week that ended Feb. 14, down more than 70% from the week that ended Jan. 10. Hospitalizations are down more than 60%.
California in late January lifted regional stay-at-home orders imposed in early December as COVID-19 numbers exploded and intensive care units were strained beyond capacity. Experts say increased vigilance around public health guidelines during that lockdown can partly help explain the drop in coronavirus numbers — especially in the Bay Area, which they say has tended to cooperate comparatively better with public health guidelines.
But the full picture of why the numbers are falling so fast is more complex, they say.
“Everyone is of course speculating that it is a function of the restrictions, behavior, perhaps combined with immunization, but I don’t know that we have the data yet that tells us exactly where and why it is happening,” said Dr. Steven Goodman, an epidemiologist at Stanford. He noted that the Bay Area is not at historically low pandemic levels, but levels that look low because of how much worse the winter months have been.
Here are four additional factors that experts say are probably at play in the major declines of the past month.
Human behavior (and the holidays)
The biggest factor driving the drop in coronavirus numbers may be that we are well past the Thanksgiving, Christmas and New Year’s holidays, with their associated social gatherings and travel, said Dr. Stephen Shortell, dean emeritus of the School of Public Health at UC Berkeley. “That has to be taken into account now that we’re into February and going toward March,” he said. “There’s been no big holidays and social events where people crowd together.”
New risks could arise in the coming months from traveling and gathering over spring break — but Shortell said it’s unlikely any associated outbreaks would be on the nationwide scale experienced during the holiday season. Instead, they would likely be more isolated events, like the recent surge at UC Berkeley that spurred a dorm lockdown after 156 people tested positive for the coronavirus.
About 12.4% of the population has received at least one coronavirus vaccine dose, and 4.9% have received both doses so far, according to CDC data on The Chronicle’s Coronavirus Vaccine Tracker. In California, 12.3% of the population has received one dose, and 4.0% of the population has received both doses so far.
Those numbers don’t explain why cases began falling in late January, when only health care workers and the oldest Californians were cleared for vaccinations. However, the continued distribution of vaccines in the Bay Area and expanding eligibility are likely to be having at least some effect on the declining numbers, experts say.
While new coronavirus strains that may be more infectious and partly evade vaccines are a concern, every individual who is vaccinated helps, said Dr. Bob Wachter, chair of the Department of Medicine at UCSF.
“The variants are the big curveball, and the great unknown,” he said. “… (But) I would bet on the vaccine winning.”
Though some research has found that coronaviruses like SARS CoV-2 tend to peak during the winter and decline after that, the concept of seasonality in relation to COVID-19 is still a bit opaque.
“I’m a little doubtful about seasonality being a major issue at the moment,” said Dr. Arthur Reingold, an epidemiologist at UC Berkeley. “To the extent that anyone thinks that there’s seasonality, the assumption would be that winter would be when we’d see the most transmission … and we’re in the middle of winter.”
In the Bay Area in particular, with its temperate climate and relative lack of weather extremes, seasonality may not be the main factor at play in the recent steep declines in coronavirus cases. But experts point out that weather does change our behavior — and with longer days and more sunshine, people more readily gather outside rather than inside, where transmission risks are far higher.
Estimates by the CDC indicate that only about 25% of people in the U.S. have been infected with the coronavirus — and given the early stage of vaccination rollout, attributing the recent plummeting coronavirus numbers to widespread immunity would be a big stretch, experts say.
“It’d be nice to think those are having an effect, but of course it’s early days for immunization, number one, and not that many people have been vaccinated yet at the community level,” Reingold said.
However, experts say seroprevalence — the number of people who have antibodies from a previous infection — may be helping block the coronavirus’ inroads, especially among populations that have been less able to avoid it, such as essential workers.
Experts caution that natural immunity may not protect against new variants, and their ability to evade vaccines is still not fully known — which are among the reasons continued protective measures like masking and social distancing are crucial.
“I think continued caution is going to be necessary,” said Shortell, noting that he plans to wear a mask and social distance probably into the next year.