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 would 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, up 1% from last week. CDC surveys show that virus levels in wastewater are also rising.
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 series of subvariants is the inevitable result of the virus looking for new mutations in its protein vanguard. – a design that is essential for Covid-19’s ability to introduce 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 found more effective tactics to evade the neutralizing antibodies found in most of our immune formulas, while still attaching to the ACE2 receptor, a protein found in human airway cells that allows it to penetrate the immune formula. . body. (Learn 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 giant 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,” Liu says. He explains that experts proposed that those 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 subvariants of Covid-19, 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 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 positioned itself in 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, there are three main factors that 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 of influenza,” says El Hussain Shamsa, an internal medicine physician at University Hospitals of Ohio, who published a 2023 study on Covid-19 trends in the year. In this case, it is believed that environmental and behavioural problems may lead to waning immunity and increased transmission of influenza viruses in winter, he says.
However, even the flu does not 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 the gathering of other people in close proximity to festivals and concerts, and the massive use of air conditioning that dries 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 suggests that June cases peaked around the week of June 17, shortly after England’s first match,” says Paul Hunter, virology representative and professor at the University of East Anglia. to recover in July as England progressed in the tournament.
But the question remains why this year-round effect is basically seen 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 have been around much longer, meaning 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. inside. (Learn more about how diseases spread when we talk and sing. )
Covid-19 has not yet entered a seasonal cycle, so we almost know when new strains will appear (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 infected in the most recent wave.
Shamsa notes that existing FLiRT variants exhibit immune evasion mutations common with the Covid-19 variants that caused the first surge in infections in the winter of 2023, allowing them to take full advantage of declining immunity levels. According to the CDC, as of July 7, only 22. 7% of people 18 and older 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 hasn’t if Covid-19 isn’t caught up over the winter, will have very little immunity to 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 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?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 perhaps it would take a year or two to get there,” says Andy Pekosz, 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 flu and other common viruses and have been exposed to them 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, this 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 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 believe that with Covid-19 still having a constant effect throughout the year, there is a need to spread more public messages about fitness to ensure that the most vulnerable people remain fully vaccinated and have access to mandatory antivirals throughout the year.
“No public fitness expert would set expectations about Covid seasonality right now,” Nash says. “If the messages were accurate, they would tell other people that they want to be informed all year round, not just in the fall and winter. ” “.
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