COVID-19 could indeed become a seasonal disease with predictable infection patterns — but it’s not there yet, epidemiologists and infectious disease experts say.
While the virus has exhibited some element of seasonality since it first emerged more than two years ago, other factors — including variant evolution, population immunity, and behavioral changes — have made seasonality less apparent.
The disease could fall into a more predictable rhythm once the population has more immunity and people return to their pre-pandemic lives, but this will likely take a few years, most experts agreed.
“There were just so many other factors driving the spread that seasonality wasn’t the primary factor yet,” said Amesh Adalja, MD, of the Johns Hopkins Center for Health Security in Baltimore MedPage today. “But as populations become more immune and people return to some semblance of their pre-pandemic ordinary lives, I think the seasonality will become more apparent.”
Michael Osterholm, PhD, MPH, director of the Center for Infectious Disease Research and Policy at the University of Minnesota in Minneapolis, noted that the U.S. has seen multiple seasons of significant COVID activity, sometimes with large peaks, such as at Delta in late summer and early fall 2021.
Surges, he said, “were really more related to when variants occur.”
Add to that the fact that immunity to infection is waning for both those previously infected and those who have been vaccinated, Osterholm said, and that makes COVID surges much less predictable than the virus the world wanted to have from the start — influenza.
Hoping for the predictability of the flu
The hope that COVID has “seasonality” may have something to do with the flu’s predictability, experts said. According to the CDC, the US can expect relatively low mortality and morbidity each year — anywhere from 12,000 to 60,000 deaths and 140,000 to 810,000 hospital admissions.
But it’s just not possible to “stick the square spigot of SARS-CoV-2 into the round hole of influenza just yet,” said James Lawler, MD, MPH, of the University of Nebraska Medical Center’s Global Center for Health Security in Omaha .
Searching for insights into the evolution of SARS-CoV-2 in the pattern of shifting and drifting influenza is not possible because they are “very different viruses,” he noted.
Influenza often drifts and requires annual vaccination, but it shifts – less frequently – and develops major changes that can lead to more virulent, pandemic influenza that causes higher mortality.
“In influenza, different segments are swapped out, resulting in massive differences in the surface protein,” Lawler said. “SARS-CoV-2 does not have the capability for such large-scale rearrangement. However, it is very slippery and mutates quickly.”
“Whether you call it shift or drift or continuous evolution of the virus,” he added, “it doesn’t seem to be slowing down.”
Experts agreed that the development of SARS-CoV-2 does indeed appear to be accelerating given what has been observed with the development of variants in the Omicron family.
“If you look at South Africa, the rate of new epidemic waves has shortened since the pandemic began,” Lawler said. “They had the Omicron surge in November/December and now 5 months later they have a BA.4/5 surge.”
This could indicate that variants evolve faster, which may not correlate with the seasons. This is happening now in the Northeast US, where BA.2.12.1 has started to launch, contributing to a surge not seen this time last year.
“Right now we have a highly infectious virus and we have ebbing immunity,” said Ali Mokdad, PhD, of the Institute for Health Metrics and Evaluation at the University of Washington in Seattle. “People are also susceptible to the virus in summer.”
That doesn’t necessarily mean we’ll see more virulent diseases — or, if you use the vicious analogy to the flu, a big “shift” causing a huge spike in hospitalizations and deaths, Adalja said. Evolution would likely favor more transmissible or immune-evasive variants, but not a variant that “keeps people in bed,” he said.
“One person in bed is a missed opportunity to infect people,” he noted. “You want people at school or at work.”
In addition, substantial T-cell immunity from previous infection or vaccination should continue to protect against serious disease, he added.
“I don’t think immunevasive variants can negate all the protection that the immune system provides,” he said. “It might be able to bypass some of the protection that antibodies confer and cause infection, but causing serious illness is a much higher requirement.”
If the Alpha variant was a “shift” from the original variety, Delta a shift from Alpha, and Omicron a shift from Delta, Adalja said, it’s possible that Omicron represents a transition to “drift,” which could lead to that it is more similar to four other coronaviruses currently circulating in the United States
Learning from other coronaviruses
These four viruses have seasonality and could provide a better model for the future of COVID-19 in the US, Adalja said. They circulate in relatively small amounts year-round, but cause spikes in infection during the winter months, when much of the country goes indoors to escape the cooler weather.
“We’ve already seen a certain element of seasonality [with SARS-CoV-2]but it’s not as obvious as with the other four coronaviruses that circulate each year,” he said.
These coronaviruses are also capable of re-infecting humans, he noted. That means SARS-CoV-2 will most likely “evolve and continue to become immune evasive to reinfect us, just like other members of the coronavirus family do.”
Lawler agreed, noting that the settlement could have helped health officials better temper expectations around COVID vaccines.
“We don’t have lifetime immunity to any of these coronaviruses, and that probably should have given us a pretty good indication that we weren’t going to get that with SARS-CoV-2 through either natural infection or vaccination,” he said. “We should have been prepared for this phenomenon of having to get a boost regularly.”
“If we had communicated so well from the start,” he added, “maybe it wouldn’t have been such a big deal to people.”
In March, CDC Director Rochelle Walensky, MD, told MPH she believed COVID was “probably” going to become a seasonal virus.
There is no doubt that for the past two years, the US has experienced its highest peaks in winter, in early to mid-January, in both 2021 and 2022.
Most experts agree that COVID-19 could be heading towards seasonality. They just aren’t ready to say that definitively given the history of SARS-CoV-2 thrown so many curveballs.
Osterholm noted that 6 months ago, Omicron had not yet reached US shores.
“We’ve never seen this subvariant activity at alpha, beta or delta, so who could have predicted where we would be 6 months ago?” he said. “That’s the challenge we have.”