As social distancing and safety measures have faded away, vacations and gatherings have returned this summer—but so has COVID.
Hospitals have been reporting a steady uptick in COVID cases after months of declining rates. The U.S. Centers for Disease Control and Prevention reported a 12.5 percent increase in COVID hospital admissions for the week of July 29, totaling 9,056 patients. National surveillance of the virus in wastewater, another primary means of tracking COVID, shows that cases have been trending upward since the last week of June. The number of new hospitalizations from COVID is nowhere near that of its peak in early 2022, soon after Omicron emerged, but clinicians and researchers say the gradually rising rates shouldn’t be taken lightly.
“The virus is still around, still causing disease, still causing morbidity and mortality. And I think with the declaration of the [U.S. federal] public health emergency being over, some people have become somewhat complacent,” says Archana Chatterjee, dean of Chicago Medical School and vice president for medical affairs at Rosalind Franklin University of Medicine and Science. “That worries me.”
While this summer’s rise in COVID hospitalizations has been mild (and cases have risen during the summer in the previous couple of years), Chatterjee says it’s an indication the virus is not following the same seasonal pattern of other respiratory viruses, which many researchers had anticipated it would. Some experts have put forward the possibility that the current upswing in hospitalizations could be a sign of a late summer COVID “wave.” But others have said it’s more of a ripple—and only time will tell if it is an indication of what’s to come.
“Fortunately, so far, we’re not really seeing a surge in hospitalizations—so people are getting sick, but most of the time those illnesses are fairly mild,” says Susan Huang, medical director of epidemiology and infection prevention at the University of California, Irvine, School of Medicine. “I don’t think we’re in our wave yet, although we all know that it’s coming. So are we at the start of a wave? Is it going to be more in the winter season? I think we’re all watching.”
Scientific American spoke with Chatterjee, Huang and other experts to further understand the driving factors behind hospitalization rates, the seasonal fluctuations of the virus and the new COVID vaccine formula that is expected to be made available in the fall.
Why are cases rising this summer?
Experts have suggested a smattering of possible reasons for the increased rates, such as relaxations in social distancing and masking, increased exposure as a result of vacation travel and waning immunity from past vaccinations. It could also be a combination of such factors.
“I think the answer is: we don’t know for sure,” Huang says.
In the two and a half years since COVID vaccines became widely available in the U.S., cases have generally tended to rise in late summer. “I think that we have seen two peaks in general, and the hospitalizations in those peaks seem to be worse in the winter,” Huang says. “So I think we’re seeing milder disease [in the summer], and I think it is very much tied to social behavior.”
As public health emergencies have lifted, people have been masking less and attending more social gatherings, which has greatly improved individuals’ physical and mental health, Huang says. She notes that with increased exposure, higher rates of COVID can, of course, be expected. But changes in social behavior and vacation travel might not be the whole story, says Helen Chu, a clinician and professor of allergy and infectious diseases at the University of Washington, who studies COVID transmission and co-leads the Seattle Flu Study.
“I don’t know that I would attribute it, necessarily, to travel. Travel can lead to introduction, but it doesn’t lead to massive outbreaks,” Chu says. “I think the fact that we are having these surges now has to do with waning immunity, mostly. Most people haven’t been boosted in at least six or nine months now.”
Immune response studies on people’s levels of antibodies—specialized pathogen-fighting proteins produced in response to a COVID vaccination or infection—suggest protection wanes at around four to six months after vaccination. For immunocompromised people, data indicate vaccine protection drops off at around three months. Less is known about T cell immunity, which tends to be a more durable and longer-lasting immune response.
The people most at risk of COVID-related hospitalization and severe disease remain those who have other medical conditions, are immunocompromised or are above the age of 65. People who are unvaccinated or have not received the currently available bivalent (double-strain) booster also have experienced an increase in severe disease. Only 17 percent of the U.S. population, or approximately 56 million people, had received the bivalent booster as of May 11 (the day that the U.S. federal public health emergency ended and the CDC stopped posting vaccination data). As fall approaches, it will be important to receive the updated booster formula, which the Food and Drug Administration selected in June.
“We know that there’s a good amount of the population that was [not] vaccinated last year and has not [been infected with] the most recent strain,” Huang says. “I think that dovetails into the conversation of what the population should consider going forward.”
How could this affect the fall booster production schedule?
The bivalent booster shot is still available to individuals in the U.S. who have not gotten one yet. Immunocompromised people of all ages can receive an additional bivalent booster two months after their first one, and people age 65 or older who are not immunocompromised can get an additional bivalent booster after four months.
During the FDA’s Vaccines and Related Biological Products Advisory Committee (VRBPAC) meeting in June, the agency made plans for a new COVID vaccine that people will be able to receive this fall to prepare for the winter season. Committee members recommended that the new formula include a strain of SARS-CoV-2, the COVID-causing virus, called XBB.1.5, which is a subvariant of the Omicron family. Unlike previous generations of the vaccine, the fall shot will not contain the original ancestral strain, which is now rarely detected in populations globally. The committee also advised switching from a bivalent vaccine back to a monovalent (single-strain) one. This is because data now suggest that a more potent single-strain shot will generate a better response than combining multiple strains in an attempt at broader coverage, Chatterjee explains.
Although the XBB.1.5 subvariant selected is no longer the dominant circulating strain, the “potpourri” of existing variants mostly remains within the Omicron lineage, Chu says. The upcoming fall booster’s XBB.1.5 formula is still expected to provide ample cross-protection against the other circulating Omicron variants, says Mark Sawyer, a pediatric infectious disease specialist at University of California, San Diego, and Rady Children’s Hospital of San Diego.
“The virus continues to evolve and develop new variants, but so far as I’ve heard, the new vaccine that’s coming this fall is going to give people a chance to top off their immunity and get in better shape for the [current] variants,” says Sawyer, who is also a temporary voting member of VRBPAC.
The shifting variant landscape also shouldn’t alter the production timeline or effectiveness of the fall booster, Sawyer says. Spokespeople from vaccine developers, including Moderna, Pfizer and Novavax, have said that their companies will be prepared to roll out an updated formula by mid- to late September—at which point the FDA and CDC will decide to authorize and recommend the vaccine for certain age groups and populations. An FDA spokesperson could not comment on exact timing but said in a statement to Scientific American that the agency anticipates making “timely action to authorize or approve” the updated vaccine.
If hospitalization rates worsen in the meantime, however, that could shake up vaccination plans.
“We’ve had little blips like this over the last year that haven’t turned into much. But if this turns out to be a blip of concern right now, we may have to start vaccinating sooner than we thought we would,” Sawyer says.
The summer cadence of COVID is also raising a larger question about the SARS-CoV-2’s seasonal patterns—and throwing a wrench in the FDA and CDC’s hope for a simpler COVID vaccine regimen that is easier for people to understand and adopt.
“We're all hoping that [SARS-CoV-2] will develop a seasonality so that it will facilitate the administration of [COVID] vaccine, along with influenza vaccine, which everybody is used to getting,” Sawyer says. “The FDA sort of, in my opinion, jumped the gun a little and said, ‘Okay, we’re going to update this vaccine every year like we do influenza and bundle those together.’ But at the FDA meeting, I think many people were far from sure that it was seasonal.”
How does the summer “wavelet” speak to the virus’s seasonality, which many experts are trying to pin down?
Seasonal viruses, such as the flu, common cold or respiratory syncytial virus (RSV), have well-known, distinct peaks and valleys throughout the year. Tracing this yearly timeline helps with managing vaccine production, hospital resources and medical staff. Most respiratory viruses spike in the winter and subside in the summer, but SARS-CoV-2 has yet to completely fall into the same pattern. “This is not yet a seasonal virus,” says Chatterjee, who is also a voting member of VRBPAC. “It is still causing disease at a time that we don’t expect respiratory viruses to cause disease; ergo, it has not entered seasonal phase yet. It is still in pandemic phase.”
Chu notes that colder temperatures and indoor gatherings in the fall and winter are associated with higher rates of respiratory illness. “Supporting that theory, we did see those other viruses go away when everyone was more or less isolating and wearing masks all the time during COVID,” Sawyer adds. Research has also shown that respiratory viruses travel through the air better in cooler, less humid spaces, and evidence suggests SARS-CoV-2 transmission is somewhat similar. The erratic rates in the past few years, however, could suggest “a different seasonality to this virus that’s a little bit broader than the other [respiratory] viruses that we usually think of as fall-winter viruses,” Chu says. “And that’s a little bit surprising.”
But she adds that this doesn’t mean COVID definitely won’t eventually settle into a more clear-cut winter season. “It may not just be at its final, steady state,” she says. Sawyer and Chatterjee say the virus’s ability to quickly evolve into new variants also presents a significant concern not only for creating and timing new vaccines but for future major waves.
“It probably is going to develop that [seasonal winter] pattern, but I think it’s premature to say it’s going to be exactly like the others,” Sawyer says. “Most of these variants last in the world’s population for a couple of months, and then a new variant takes over. That’s different than influenza and other viruses, and that may disrupt the seasonality tendency of [a higher number of cases] in the winter. If we get a whole new variant that’s completely different than what we’ve been dealing with, then it may take off right now and cause another big surge.”
Whether this current little summer bump could spike or simmer, we’ll have to wait but remain vigilant, he says. Huang is also keeping an attentive eye on the situation as winter approaches, especially given the low number of people who have gotten the bivalent booster. She hopes the coming fall vaccine campaign will be more successful.
“I think that whether or not we have a summer wavelet, at least we should be prepared to do some preventative activities in the fall,” she says. “Hospitalizations are a great kind of sentinel marker if we’re going to be seeing [changes in disease]. Unfortunately, by the time you wait for that marker, vaccinating and having a campaign is a little late. So I think that’s why this fall becomes really important to try to protect people.”