Every July for the past four years, epidemiologists at the US Centers for Disease Control and Prevention (CDC) have seen a surge in Covid-19 cases and hospitalizations, an annual trend that It has been called “the summer surge. “
This summer, in the United States, Covid-19 rates are reported to be highest in Arizona, California, Hawaii, and Nevada. In those western states, the number of positive tests reached 15. 6% in the week ending July 6, a 1% increase. last week. CDC surveys show that virus levels in wastewater are also in decline.
A shift occurred on the other side of the Atlantic, where, according to the United Kingdom Health Security Agency, positive Covid-19 tests rose from 4% at the end of March to 14% at the end of June.
The recent buildup has been attributed to the FLiRT subvariants, the most recent evolution of the Omicron strain of Covid-19 that emerged in late 2021. This new set of subvariants is the inevitable result of the virus looking for new mutations in its spike protein. . – a design that is imperative for the ability of Covid-19 to enter human cells, but which can also alert the human immune formula to the presence of the virus in the body. As a result, Covid-19 has discovered more effective tactics to evade the neutralizing antibodies found in most of our immune formula, while still latching onto the ACE2 receptor, a protein found in human airway cells that allows it to enter the immune body. (Find out how Covid-19 symptoms have been replaced as new variants have emerged. )
According to Shan-Lu Liu, who directs the Emerging Viruses and Pathogens Program at Ohio State University and has studied FLiRT subvariants, those newer Covid-19 viruses have managed to strike a balance between evading the immune formula and proceeding to bind. cells, which is the cause of a gigantic number of new cases.
“Older people and the immunocompromised are especially vulnerable to new subvariants due to their weakened immune reaction to vaccination and herbal infections,” Liu said. He explains that experts proposed that these teams receive booster shots, adding the monovalent XBB. 1. 5 vaccine. Take aim at the Omicron XBB 1. 5 subvariant of Covid-19, which appeared in 2022.
New Covid-19 subvariants are finding new tactics to evade the immune formula (Credit: Getty Images)
Given the need to protect the most vulnerable in society, the study of these frequently emerging variants remains very important to update Covid-19 vaccines around the world. Based on work by Liu and others, US regulators and the World Health Organization have been able to make express recommendations related to new vaccine targets based on the most recent mutations in the Spike protein, in time for launch. of the latest Covid vaccine in the fall.
Still, for scientists who follow the evolution and changes of SARS-CoV-2, it is still highly unlikely that they will know when the next major strains will appear. While more common respiratory infections such as influenza or respiratory syncytial virus (RSV) if we stick to seasonal patterns, extending the fall or winter months before tapering off in the spring and summer, Covid-19 has yet to position itself in such a singular cycle.
In the wake of the latest summer outbreak, it remains to be seen whether Covid-19 will one day become a truly seasonal virus and, if so, how long it will take.
According to epidemiologists and medical researchers, three main factors cause cases of infectious diseases: the virus itself, the number of vulnerable humans at any given time, and the situations in which the virus spreads.
“Seasonality is a characteristic shared by many viruses, the most observed being the endemic annual winter flu,” says El Hussain Shamsa, an intern at Ohio University Hospitals, who published a 2023 study on COVID-19 trends this year. In this case, it’s thought that environmental and behavioral problems may lead to lower immunity and increased transmission of influenza viruses in the winter, he says.
However, even the flu doesn’t go away completely. Akiko Iwasaki, a professor of immunobiology at Yale University, says the summer wave of Covid-19 will likely be partially exacerbated by points like gatherings of other people near festivals and concerts, and the heavy use of air conditioning that dries out the air and encourages viral spread.
As an example, experts on infection transmission in the United Kingdom recommend that the number of people gathering in the gardens of crowded pubs and bars to watch this summer’s Euro 2024 football tournament is likely to increase. the cause of many recent cases of Covid-19 in the country. “The most recent data suggest that June cases peaked around the week of June 17, shortly after England’s first match,” says Paul Hunter, a virology representative and professor at the University of East Anglia. “Cases started to rise in July, as England progressed through the tournament. “
But the question remains as to why this year-round effect is mainly seen in Covid-19 and not in other respiratory infections. According to scientists, this is due to the fact that population immunity is much higher than that of many of the same old seasonal viruses. such as influenza, rhinovirus and RSV. One explanation is that they’ve been with us much longer, which means they want more ideal situations to infect us, which only happens during the fall and winter months, when temperatures drop and schools return. , and other people gather more indoors. (Learn more about how diseases spread when we speak and sing. )
Covid-19 has not yet entered a seasonal cycle, so new strains are almost expected to emerge (Credit: Getty Images)
Because Covid-19 is still a relatively new virus, our sterilizing immunity (the body’s ability to eliminate a pathogen before it has a chance to start replicating) is significantly weaker. Scientists say this phenomenon is exacerbated by low vaccination rates, which make population immunity dependent on the number of people inflamed in the most recent wave.
Shamsa notes that existing FLiRT variants feature immune evasion mutations common with the Covid-19 variants that caused the first surge in infections in winter 2023, allowing them to take full advantage of falling immunity levels. According to the CDC, as of July 7, only 22. 7% of people over the age of 18 in the United States were up to date on their Covid-19 vaccines, compared to 48. 2% for the flu. As a result, Hunter says anyone who did not contract Covid-19 over the winter will have very little immunity to the FLiRT variants, which is causing the current rise in cases.
“With Covid-19, I think a lot of other people just don’t want or think they want the vaccine, however, what that means is that population immunity rises and falls almost in unison based on the frequency and intensity of recent past surges. “, explains Denis Nash, professor of epidemiology at the City University of New York.
So will Covid-19 ever evolve into a more seasonal trend as human exposure to the virus increases? In part, this trend is already taking shape, and Hunter noted that the increase in cases, hospitalizations and deaths in the summer is much more moderate than that seen in December and January. In the United States, another 327 people died from Covid-19 during the week of June 15, compared to 2,578 during the week of January 13.
“We may never reach a point of population immunity to SARS-CoV-2 that reduces summer cases to zero, or perhaps it would take a year or two to get there,” says Andy Pekosz, professor of molecular microbiology and immunology. at the University of Pennsylvania. Johns Hopkins University.
However, Nash predicts that this procedure may take decades or longer, noting that humans have lived and been exposed to influenza and other common viruses for many years.
Scientists say population immunity is much higher with pre-existing seasonal viruses than with Covid-19 (Credit: Getty Images)
If Covid-19 discovers a robust seasonal pattern, the question arises as to where an annual peak would fall between the peaks we already see for RSV, which peaks in early fall, and for seasonal flu, which peaks in mid-winter. Array Pekosz describes the concept of “viral interference,” whereby pathogens evolve in cycles over time that prevent many viruses from circulating at once. “In fact, the former would infect other people and cause an era of non-specific immunity against other viruses,” he explains.
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These are all questions that would possibly take years, if not decades, to adequately answer. However, some scientists suspect that Covid-19 will probably never completely disappear from the summer seasons, due to the innate properties of coronaviruses that allow them to continue circulating. even in hot weather.
“We’ve known for a long time that human coronaviruses are as seasonal as the flu, which is actually favored by cold temperatures and drought,” says Bill Hanage, a Harvard epidemiologist. “In fact, before 2020, we used to call coronavirus infections bloodless summer because they were less visibly geared toward bloodless months, so it’s not a surprise. “
Whatever happens, for now, experts say that with Covid-19 still having an ongoing effect year-round, more public messaging about fitness is needed to ensure the most vulnerable people remain fully vaccinated and have access. to mandatory antivirals. all year.
“There’s no need to worry about the fitness experts in the public setting expectations about Covid seasonality right now,” Nash says. “If the messages were accurate, they would tell other people that they want to be informed year-round, not just in the fall and winter. “
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