Covid-19 could eventually be seasonal, scientists say
By Jacqueline Howard, CNN
(CNN) -- The United States has been through waves, spikes and surges of Covid-19 throughout the pandemic, and now there is concern that the nation may be entering a new winter surge.
Sometimes scientists know which factors drive a new rise in cases, but some surges have been inconsistent and hard to predict. Eventually, scientists suspect the rise and fall of coronavirus infections could shift into a more typical seasonal pattern.
Early next year, health officials plan to begin serious talks about what the pandemic's end might look like and how will we know when we've reached that point.
The US isn't there yet.
As of Thursday, the US is averaging 121,084 new Covid-19 cases each day, according to data from Johns Hopkins University. This is 62% higher than a month ago.
Though Covid-19 cases and hospitalizations in the United States dropped off at the tail end of a summer surge, they've risen over the past few weeks. More than half of new hospitalizations over the past month have been in Midwestern states, especially Michigan and Ohio.
Dr. Laolu Fayanju and his colleagues have treated a rotating door of Covid-19 patients year round. Now, once again, they're bracing for a possible winter surge.
"We have seen an increase in our numbers in just the last month across all 11 of our centers in northern and southern Ohio," Fayanju, an Oak Street Health physician based in Cleveland, Ohio, told CNN.
"So, we are seeing an increase."
Scientists have observed "waves" of coronavirus infections during the pandemic that ebb and flow across regions of the United States -- but the factors driving these patterns of infections are complex.
While no one can predict the future, "we're living through an intra-Covid world, and in a post-pandemic Covid world, I think what we would see is an endemic infection, not unlike the seasonal flu," Fayanju said.
Endemic means that a disease has a constant presence in a population -- but it's not overwhelming health systems or affecting an alarmingly large number of people as typically seen in a pandemic.
Some scientists point to human behaviors, such as travel, as fueling the waves.
Some think the waves are more evidence that Covid-19 is on its way to eventually becoming a seasonal endemic disease, with more cases occurring in the cold winter months at times when outdoor temperatures drop and people gather indoors.
Others argue that seasonal waves of Covid-19 could be more complex, since during the pandemic there have been both expected winter surges and less-expected summer surges, too.
"We need more research to disentangle all the factors that may link seasonality to Covid-19 cases," Dr. Hawre Jalal, assistant professor at the University of Pittsburgh, told CNN.
But there is one thing many scientists agree on -- coronavirus infections in the future could follow seasonal cycles.
'There will always be this underlying seasonal rhythm'
"Seasonality is real," said Dr. Donald Burke, professor and former dean of the University of Pittsburgh Graduate School of Public Health.
"We think that it will be seasonal for the foreseeable future," Burke said.
One study by researchers in Spain, published in October in the journal Nature Computational Science, classifies Covid-19 as a "seasonal low-temperature infection."
Separately, Burke and his colleague Jalal co-authored two preprint papers that describe the seasonal patterns of the pandemic's waves so far and suggest that these patterns may repeat in subsequent years. The papers have not published in a peer-reviewed journal but were posted online to the server medrxiv.org in July and November.
For the first paper, the researchers tracked Covid-19 case counts in the United States, Mexico and Canada from early 2020 to May 2021 and built animated maps that illustrate how many Covid-19 cases and deaths were recorded where and when -- visualizing trends in the data and revealing patterns. The visualizations showed four dominant waves that occurred from March 2020 to May 2021.
The visualizations showed that the first wave of the pandemic in early 2020 was largely in the Northeast region of the United States -- especially when a surge of cases hit New York City. Then, in the summer, a second wave of infections involved the South and traveled northward to the upper Midwest.
"It contradicted the notion that this should be a northern, cold season disease," Burke said.
Then in the fall, wave three started in the Dakotas before spreading throughout North America and then wave four followed, with cases surging again in the winter in the Northeast, South and West.
In 2021, despite the authorization of coronavirus vaccines and the emergence of the Delta variant, the spread of Covid-19 cases was somewhat similar to the patterns seen in 2020 -- even though the number of cases and magnitude of changes was not as great in 2020, the patterns followed a similar seasonal path.
For the second paper, the researchers analyzed the waxing and waning patterns of reported Covid-19 cases the United States, Mexico and Canada from January 1, 2020 through Oct 31, 2021 -- an additional five months of data compared to the first paper.
Now in early December 2021, Covid-19 case counts continue to rise. For the first time in two months, the United States is averaging more than 100,000 new cases each day, shortly after millions of Americans traveled for the Thanksgiving holiday.
"Our modeling of this suggests that the rhythm of the future epidemics will be seasonal, but that the amplitude may vary from year to year or time to time. There will always be this underlying seasonal rhythm, being with an extra half beat in the South, but certainly the northern wave in the wintertime," Burke said.
"That will be modified depending on what proportion of the population is vaccinated, whether or not a new strain is coming in at that moment, and so the amplitude will change," he said. "But the basic rhythm will probably be the same."
Biden's Covid-19 winter plan
Just last week, US President Joe Biden warned of a potential winter surge of Covid-19, announcing a new strategy aimed at fighting the surge without enacting unpopular lockdowns as the pandemic approaches its two-year mark.
"It doesn't include shutdowns or lockdowns, but widespread vaccinations, and boosters, and testing and a lot more," Biden said Thursday. He acknowledged a likely rise in cases over the coming weeks, as weather turns colder in much of the country and people begin to gather more indoors.
Biden called for a multipronged approach, with a heavy emphasis on expanding vaccinations to the remaining Americans who have resisted getting shots, and to provide boosters to the now-eligible population of all adults. The Biden administration also now requires insurance companies to pay for at-home tests and has changed international travel rules to require travelers flying into the US from another country to test negative for Covid-19 one day -- not the previous three days -- before their departure.
Scientists -- including Jalal of the University of Pittsburgh -- have warned that the United States is likely entering another winter surge of Covid-19 right now.
"Since it has been doing it twice so predictably, it's highly likely that a winter wave will happen again," Jalal told CNN.
"That doesn't mean that we should give up and say, 'It's seasonal, we just have to go with that.' I think a very important distinction to make is that we have some predictable pattern to it, so we can prepare for it," Jalal said. "You can make the public health services available before the wave starts."
Even though Covid-19 could become seasonal in the future, it is important to remember that the world is still grappling with a pandemic right now. We have not yet entered an endemic phase, said Sen Pei, assistant professor of environmental health sciences at Columbia University's Mailman School of Public Health.
So, while seasonality certainly could play a role in an upcoming winter surge this year, so could low vaccination rates and the spread of the highly transmissible Delta variant -- which dominates in the United States and around the world -- and the newly identified Omicron variant.
"People should take precautions during the winter," Pei told CNN.
He says this surge may be caused by several factors. "The first one will be the emergence of the new Omicron virus. I think that's the most important factor right now. And the second one will be seasonality of the virus and impacts of climate factors like temperature, humidity and also human behaviors -- people will gather more indoors during the wintertime."
Pei added that vaccination and booster shots "will also play an important role in reducing severe disease outcomes."
Yet once the coronavirus becomes endemic -- and case rates, hospitalizations and deaths fall to very low numbers -- the United States could see more pronounced seasonal patterns in infection rates than what are occurring now.
"I think still we are still far away from that," Pei told CNN.
"We will not enter into an endemic phase until the large majority of the population has immunity to the virus either from infection or from vaccination," he said. "The case fatality rate is still very high, much higher than flu, and a large proportion of the population still do not have immunity."
When will the coronavirus become endemic?
State and local health departments plan to meet with the CDC in the new year to discuss what type of data or metric will be needed to determine that the coronavirus pandemic has ended and shifted into an endemic phase, Lori Tremmel Freeman, chief executive officer of the National Association of City and County Health Officials, told CNN on Wednesday.
"We plan to begin having listening sessions in early January to talk with jurisdictions and their health officials about what we need to be thinking about to transition from pandemic to endemic. The idea is to envision what this looks like long-term and what metrics and considerations would be utilized to make the determination," Freeman said.
As Covid-19 transitions into an endemic disease, any seasonal patterns the virus might follow warrant discussion, Freeman added.
"We would all like to view that as a possibility -- where we're just tackling the season and trying to manage one season over the next in terms of severity and other aspects of disease transmission," Freeman said, calling it "crucial" to discuss long-term vaccination and mitigation plans.
"There are still many unknowns that it sometimes feels premature and overwhelming to predict what might happen to even make plans, but it is necessary."
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