It will happen, sooner or later. It will happen that Covid-19 will become a seasonal disease, little more than a cold for some, or a more dangerous illness like the flu for others. But still a manageable matter, an integral part of our lives, not too impacting on everyday life, on affections, on the economy. At least – we must not forget – in the rich half of the world. “Sars-CoV 2 will become an endemic virus, I’m sure. The point is to understand when”, confirms, in fact, Pierluigi Lopalco, full professor of Hygiene at the University of Salento. And when will depend on many factors.
The health to come
To understand how long the road to reach this goal will be, it is first of all good to clarify what is meant by endemic. “We can define endemic as a virus that spreads in a population constantly over time, without presenting particular frequency peaks”, continues Lopalco. An endemic disease can however present the so-called epidemic recurrences, ie when the virus occasionally generates outbreaks or epidemic increases and then re-enters a “normal” circulation among the population. Most viruses behave in this way, so much so that they are defined as “seasonal” to the extent that they do not have much circulation during the summer or spring and instead become more evident in the autumn and winter. A classic seasonal endemic disease is, for example, the cold: a coronavirus – not surprisingly – which is always in circulation but which when it is cold and you spend more time indoors has a greater chance of being transmitted by an infected individual to the other, thus creating outbreaks.
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Whether Sars-CoV 2 will behave more and more like a seasonal virus is still a matter of debate. If on the one hand it is clear, as he points out, on The Scientist, Sen Pei, epidemiologist at Columbia University in New York, that we do not have to rely on the climate to hope it disappears, on the other hand it is likely that the climate factor is one of the pieces to be taken into consideration when we try to complete the great puzzle of the spread of Covid-19 .
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In an article published in Nature in June of this year, Pei and his colleagues estimated, for example, that the combined effects of a temperature between 20 and 40 degrees, low humidity and low ultraviolet radiation were able to explain 17.5% of Sars-CoV 2 transmission in 2,669 US counties from March to December 2020. These climatic conditions were, in short, associated with RT, meaning the average number of new infections caused by a single person infects, given the public health measures in place, in a population where all are presumed to be susceptible. In addition to the weather, however, everything else determines the spread of the virus, namely the containment measures adopted by the various governments, from lockdown to remote teaching, from social distancing to mandatory masks and so on.
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A study instead conducted by Xavier Rodò, climatologist at the Barcelona Institute for Global Health who has long been studying the relationship between climatic factors and the spread of diseases such as cholera, malaria and influenza, has identified a strong correlation between temperature, humidity and cases of Covid-19, especially in the first two pandemic waves in Lombardy, Thuringia (Germany) and Catalonia. According to Rodò, the virus seems to spread more easily in conditions that we define as winter: low temperatures, little humidity and little sun. But the same researcher points out that also in this case the influence of meteorological factors is only the tip of the iceberg. The bulk of it is mainly the behavior of people, given the nature of the disease and the way in which the virus is transmitted. However, at least at certain latitudes (essentially in temperate climates), climatic conditions could over time become one of the main factors to be considered to improve disease containment, provided that transmission levels in the population are low.
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In the eventual endemization process, however, the main role will be played by us, understood as people able to slow down the circulation of Sars-CoV 2 through vaccination. “As long as the population is fully susceptible and immunologically uncovered, the virus finds virgin ground, so to speak, and spreads in great pandemic waves across the planet,” continues Lopalco. But over time we will see cyclical waves less and less evident, and the spread will be similar to that of all the respiratory viruses we know. “Even before Covid-19 we had to deal with viral pneumonia every year. And pulmonologists and infectious disease specialists knew it well: there were years in which they increased, and others in which few cases occurred, according to a typical endemic circulation of the virus. “, adds the expert. The surge in infections is due to the fact that a population susceptible to the virus accumulates over the years, or because a mutated strain emerges which then encounters more people who have lost immunity on its way. Basically this is what happens today with the variants of Sars-CoV 2.
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In this sense we can talk about endemic when we reach a balance between the number of immunized people (naturally or thanks to the vaccine) and the mutations of the virus. “This balance implies that the emergence of a slightly more contagious strain will infect that small portion of non-immune people (that is, who have not had the disease and have not been vaccinated) and part of that large portion who have had the disease. or she has been vaccinated, “adds Lopalco. We know: neither natural infection nor vaccination guarantees one hundred percent coverage. But it is easy to understand that if we have one hundred percent of people who are artificially or naturally immunized, the circulation of the virus is slow. And this is our goal: to slow down. That is, says Lopalco, “to ensure that out of ten people who come into contact with the virus, only one has partial protection, or two who, however, will be contagious for two days instead of ten. This means slowing down the circulation of the virus “.
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So when can we say that the epidemic has become an endemic? According to Joshua Petrie, epidemiologist at the University of Michigan School of Public Health, in some regions of the United States the disease Covid-19 could be classified as endemic as early as 2022. But Sars-CoV 2 certainly does not stop at borders, and we must do the accounts with areas of the planet where vaccination coverage is still too low. “The slowdown in circulation depends on the relationship between the number of immunized people and the mutations of the virus”, continues Lopalco. If the circulation of the virus is fast, the probability of generating new variants is much higher in the unit of time. It is a probabilistic fact: if there are millions of infections in a given unit of time, it is clear that the likelihood of a more contagious variant emerging becomes concrete. If instead of a million there are hundreds of thousands, the probability of a variant emerging is greatly reduced. So you can help the progression towards endemia only with vaccination (or with the natural infection, which however leaves behind a trail of deaths), waiting for the virus to make its history.
But the future of Sars-CoV 2 will also depend on another factor: the animal tank. We already know that the virus probably originated in bats, and that it passed to humans through an intermediate host. We also know that it can infect several species, including cats, rabbits and hamsters. It is particularly infectious in minks, and mass outbreaks on Danish and Dutch farms have resulted in a killing of animals. “If the virus were to settle in a population of wild animals and from there it could move to humans it would become very difficult to control,” comments for example the American epidemiologist Michael Osterholm, who joined President Biden a year ago. against Covid-19: “There is no disease in the history of humanity that disappeared from the face of the Earth when the animal reservoir played such an important role in transmission,” he adds.
In short, the road to endemic is difficult to predict, but the human species can direct it or make it shorter. The different countries, continues the American epidemiologist, can reduce the transmission of the virus with containment measures and at the same time vaccinate as many people as possible. If we let the virus circulate undisturbed, then, Osterholm concludes, the worst moment of the pandemic will always be ahead of us.
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