COVID is down sharply in CT. No one knows exactly why.

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Connecticut is seeing a sharp and sustained decrease in the coronavirus.

The seven-day average rate of positive tests is down near 2 percent, from a high of 7.4 percent in the second week of January. The number of people in hospitals in the state with COVID-19 fell to 568 on Thursday, less than half the total of just a few weeks ago.

The number of daily deaths, averaging about 20 over the last week, is half what Connecticut saw one month ago.

The question is, why the decline?

“It is a good question without a clear answer,” said Pedro Mendes, a disease modeler for UConn’s Center for Quantitative Medicine.

There are a few possible factors to consider including broader immunity, especially in nursing homes; the time of year; the effects of all the mask-wearing and social distancing; and, some experts say, the unexplainable cycles of viruses that seem to have a mind of their own.

Whatever the reasons, Gov. Ned Lamont has responded by loosening restrictions in the shutdown.

“The numbers are all trending in the right direction and I’m not sugarcoating it,” Lamont said during his Thursday briefing. “It’s good news.”

Immunity, but not the whole herd

The first possible cause is immunity, though that raises questions. “We can assume, with a high degree of certainty, that this is not yet herd immunity,” Mendes said — meaning the immunity may be in key groups, not the whole state population.

It’s possible that older adults and nursing home residents might be seeing a measure of vaccine-conferred immunity, Mendes said.

There were 483 coronavirus cases in Connecticut nursing homes on Jan. 15. On Thursday, the number was 30, a dip likely attributable to the high rate of vaccination in nursing homes. But that wouldn’t account for the steep drop in infections the state is seeing, according to Mendes.

“For the general population, there isn't yet much ‘herd immunity’ effect,” he said. “I think that will start having an impact when we get closer to 50 percent vaccination.”

Catching the virus does confer some immunity, though how much and for how long is “very variable,” Mendes said, depending in part on the severity of infection.

Connecticut has so far administered at least one dose of vaccine to 14 percent of the population. As of Thursday, 271,903 people in Connecticut had tested positive for the coronavirus, but Richard Martinello estimates that as many as 25 percent of the state’s 3.56 million residents have been infected.

That means there have been about three cases for every one confirmed medically, a ratio many epidemiologists have cited. Still, said Martinello, medical director for infection prevention at Yale New Haven Hospital, “We have a long way to go before we achieve herd immunity.”

The season of declines

There’s also the question of winter. The cold, snowy weather may be helping to encourage social distancing guidelines.

“We're, quote, ‘flattening the curve’ with the use of non-pharmaceutical interventions,” Martinello suggested.

Seasonality is a possibility. Science is still not sure if infections from the coronavirus are rooted in seasonal weather shifts, what Martinello called “still a big unknown.”

“It's too early to call,” he said. “We're going to have to wait and see when this pandemic settles out, when we've achieved herd immunity, then we have to study the epidemiology of the virus to better understand how or how not seasonality impacts it.”

Within the question of seasonality may be the best available answer to the question of why COVID-19 numbers in Connecticut — and in the rest of the country — are dipping so deeply.

There is, Martinello said, a natural ebb and flow to diseases like the coronavirus. They come and go in waves, and “there's really not a good understanding of why waves occur,” he said.

Connecticut is on the back end of its second coronavirus wave. There was a peak toward the end of April, and then some respite until the fall, followed by a peak in December or January — depending on what we were measuring. Hospitalizations, for example, had a recent peak of 1,269 on Dec. 15, but remained above 1,100 until the third week of January.

“I think the slowing down of cases is mostly due to the ‘holidays effect’ being over now,” Mendes said. “A large factor in this recent surge was travel and higher risk-taking in the Thanksgiving-Christmas-New Year season. We're now over a month after that and that's why we are seeing lower cases.”

Why those waves occur, and why they do not seem to match with the shift in seasons, is not fully known, according to Martinello. It’s not specific to just COVID-19, but a dynamic seen in most respiratory diseases.

“Why do we see it go away, come back, and then go away?” he asked. “Typically, if we didn’t have vaccination, these waves would continue, presumably, until we achieve that degree of herd immunity.”

No solace yet

The bad news is, if we don’t see huge swaths of people vaccinated, it’s possible Connecticut could see a third wave.

“If we're seeing the downslide of the second wave, this is the time to get vaccination out,” Martinello said. “So that when and if that third wave does start, we have a much more protected population, so we're going to really mitigate the impact of that third wave.”

The United States as a whole is seeing a clear decline in coronavirus. It’s hard to compare states because of different testing and tallying methods, but Connecticut’s hospitalization rates are about the same as the nation’s and the state’s positive test rate — a key measure of community spread — is significantly lower.

In Connecticut, there has been one reported case of the B.1.351 coronavirus variant initially identified in South Africa, and 42 cases of the B.1.1.7 variant first found in the United Kingdom, both of which are potentially more transmissible than the original strain.

“We shouldn't take too much solace yet,” said Andy Slavitt, a senior adviser to President Joe Biden’s COVID-19 response team, who appeared on Lamont’s briefing Thursday and specifically expressed concern about the two variants.

“What happens with these variants, they tend to grow before you can see that they're growing,” he said.

But that, Mendes said, is why it’s so important to vaccinate as many people as possible.

“The good news is that the lower we can get in infections and hospitalizations before the B.1.1.7 starts becoming widespread, the better we can weather the likely new surge of infections that will cause,” Mendes said.