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 decreasing.
A shift has taken place 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 accumulation has been attributed to 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 seeking new mutations in its beak. . – a design that is essential for the ability of Covid-19 to enter human cells, but that can also alert the human immune formula about 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 binding to the ACE2 receptor, a protein found on human airway cells that allows entry into the immune formula. body. (Find out how Covid-19 symptoms have replaced each other 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 to the cells. which is the cause of a gigantic number of new cases.
“Older and immunocompromised people are especially vulnerable to new subvariants due to their weakened immune reaction to vaccination and natural infections,” says Liu. She explains that experts proposed that these groups receive booster shots, adding the monovalent XBB. 1. 5 vaccine. 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 for updating Covid-19 vaccines around the world. Based on work by Liu and others, U. S. 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 the release of the latest Covid vaccine in the fall.
Still, for scientists following 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 the most common respiratory infections such as the flu or respiratory syncytial virus (RSV) If we stick to seasonal patterns, extending into the fall or winter months before tapering off in the spring and summer, Covid-19 has not yet has settled into such a unique 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 annual winter endemic flu,” says El Hussain Shamsa, an internal medicine physician at Ohio University Hospitals, who published a 2023 study on COVID trends. -19 this year. In this case, it is thought that environmental and behavioral problems can lead to decreased 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 such as gatherings of other very close people at festivals and concerts. and the widespread use of air conditioning that dries out the air and encourages viral spread.
As an example, infection transmission experts in the United Kingdom recommend that the increase in the number of people gathering in crowded gardens and bars to watch this summer’s Euro 2024 football tournament is most likely the cause of many recent Covid cases. -19 in the country. ” The latest 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. to bounce back in July as England progressed through the tournament. “
But the question remains as to why this year-round effect is basically observed with Covid-19 and not with other respiratory infections. According to scientists, this is because population immunity is much higher than that of many common seasonal viruses, such as influenza, rhinovirus, and RSV. One explanation is that they’ve been around for much longer, which means they want more ideal situations to infect us, which only happens during the fall and winter months, when temperatures drop, 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 lower. 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 common immune mutations with the Covid-19 variants that caused the first surge in infections in winter 2023, allowing them to take full advantage of declining immunity levels. According to the CDC, as of July 7, only 22. 7% of people over 18 in the United States were up to date with their Covid-19 vaccinations, compared to 48. 2% of those in the flu. As a result, Hunter says anyone who hasn’t caught Covid-19 over the winter will have very little immunity to the FLiRT variants, which are driving the existing wave of cases.
“With Covid-19, I think a lot of other people just don’t want or think they want the vaccine, but that means population immunity is rising and falling almost in unison depending on the frequency and intensity of recent past waves. “says 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? Some say this trend is already emerging, and Hunter noted that the increase in cases, hospitalizations and deaths in the summer is much more moderate than observed. in December and January. In the United States, another 327 people died of 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 maybe it would take a year or two to get there,” says Andy Pekosz, a professor of microbiology and molecular immunology. at Johns Hopkins University.
However, Nash predicts that this procedure may take decades or even longer, noting that humans have been living with and exposed to the flu and other common viruses for many years.
Scientists’ 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, it begs the question of where an annual peak would be between the peaks we already see for RSV, which peaks in early fall, and for seasonal influenza, which peaks in mid-winter. Pekosz describes the concept of “viral interference,” which means that over time, pathogens evolve in cycles that prevent many viruses from circulating simultaneously. “In fact, the former would infect other people and cause an era of non-specific immunity opposed to other viruses,” he explains.
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These are all questions that would likely take years, if not decades, to properly 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 some time that human coronaviruses aren’t as seasonal as the flu, which is favored by cold temperatures and droughts,” says Bill Hanage, an epidemiologist at Harvard. “In fact, prior to 2020, we used to call coronavirus infections bloodless summer because they were less visibly skewed toward bloodless months, so this is not a surprise. “
Whatever happens, for now, experts say that since Covid-19 still has a consistent effect throughout the year, more public messages about fitness are needed to ensure the most vulnerable people remain fully vaccinated and have access. to the mandatory antivirals. all year round.
“No public fitness expert would set expectations about Covid seasonality at this point,” 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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