Will COVID-19 become a seasonal infection?

Infectious diseases are caused by external causes - bacteria, viruses, parasites or fungi. Many of them are characterized by seasonality - outbreaks occur at the same time of the year. For example, influenza comes to the Northern Hemisphere with Global Patterns in Seasonal Activity of Influenza A / H3N2, A / H1N1, and B from 1997 to 2005: Viral Coexistence and Latitudinal Gradients every winter (some epidemiologists directly call winter “flu season”), and outbreaks Chickenpox is more common RECURRENT OUTBREAKS OF MEASLES, CHICKENPOX AND MUMPS: I. SEASONAL VARIATION IN CONTACT RATES in spring.

Noncommunicable diseases cause all other causes, from genetic problems to injury, they are not contagious. Such diseases can be massive, but they do not strictly depend on the season. For example, 17.9 million people die annually from cardiovascular diseases, but they do not have pronounced peaks in one season or another.

What does the weather affect

Infectious diseases can be compared with each other in three parameters, which are weather dependent Seasonal infectious disease epidemiology.

Vitality of the pathogen

The causative agent of cholera - Vibrio cholerae - is able to survive for months by Environmental reservoirs of Vibrio cholerae in stagnant water, and influenza virus particles, hitting, for example, banknotes, keep Survival of Influenza Virus on Banknotes infectious for only one to three days. Although after this period, viral particles from banknotes do not disappear anywhere, during this time they have Mechanisms by Which Ambient Humidity May Affect Viruses in Aerosols the capsid (viral envelope) becomes unusable, and the virus cannot infect anyone.

Climatic factors (temperature, humidity, amount of sunlight) and non-climatic (pH and salinity of water) can both prolong the life of pathogens and accelerate their death. For example, the stability of the influenza virus is influenced by the Global environmental drivers of influenza temperature and humidity. In countries with temperate climates, the virus survives best in winter, and loses ground by spring. Influenza outbreaks are not seasonal in tropical climates.

The survival rate of Vibrio cholerae in water is influenced by Influence of Water Temperature, Salinity, and pH on Survival and Growth of Toxigenic Vibrio cholerae Serovar O1 Associated With Live Copepods in Laboratory Microcosms and its pH and salinity. The bacteria thrive best at an alkaline pH of 8, 5 and a salinity of 15 percent. If the water becomes more acidic and less salty - for example, due to the vital activity of some algae or heavy rains - the vibrio dies faster and is less likely to infect someone.

Contagiousness, that is, infectiousness

When assessing the rate at which a disease spreads, epidemiologists use the R metric ₀ - This is the average number of people who can catch the disease from one sick person. Measles, for example, is highly contagious: one patient infects the basic reproduction number (R0) of measles: a systematic review of 12 to 18 people. Flu is ten times weaker Modeling influenza epidemics and pandemics: insights into the future of swine flu (H1N1), its R ₀ - 1, 4–1, 6.

Elena Burtseva, Head of the Laboratory of Influenza Etiology and Epidemiology at the Institute of Virology of the Gamaleya Research Center for Chemistry, noted in a conversation with N + 1 that the rise in the incidence of many acute respiratory viral infections is also associated purely with social factors: the vacation period ends, children return to school. That is why an increase in the incidence of ARVI from year to year is recorded from mid-September to early October.

The second human factor that theoretically can Factors Influencing the Seasonal Patterns of Infectious Diseases affect disease outbreaks is the characteristics of the human immune system, depending on the season. For example, with the onset of cold weather, we spend less and less time outside and wear clothes that cover the body. As a result, less ultraviolet radiation gets on the skin and the synthesis of vitamin D decreases in the body, which plays an important role in protection against bacterial and viral infections. However, there is empirical evidence that people who take this vitamin in pills get the flu. Shortcomings of Vitamin D ‑ Based Model Simulations of Seasonal Influenza are no less common than those who do not drink vitamins.

Transfer method

Some diseases are transmitted directly, and some - indirectly. What You Need to Know About Infectious Disease flu and SARS are transmitted directly from the source, which spread from a sick person to a healthy one.

The West Nile virus, which travels from person to person in the stomach of a mosquito, and African sleeping sickness, which is transmitted by the tsetse fly, are indirectly transmitted. The latter actively reproduces in THE ECOLOGY OF AFRICAN SLEEPING SICKNESS in the rainy season, and in addition, Epidemiology of human African trypanosomiasis lives from three to five months versus one or two in the dry season. At this time of year, flies become more and more often they bite people - here's an outbreak of sleeping sickness. The same applies to tick-borne encephalitis, says Burtseva: ticks wake up in early spring, and it is in the spring that the peak of diseases is recorded. And the second wave is recorded in autumn - and this is due to the life cycle of ticks.

The coronavirus disease (COVID-19) pandemic in some of its manifestations is very similar to the respiratory diseases we know, so many researchers use Successful containment of COVID-19: the WHO-Report on the COVID-19 outbreak in China to model SARS or influenza outbreaks. predicting outbreaks of COVID-19.

Coronavirus disease came to us in winter. Before asking the question of whether it is worth now waiting for its end in the summer and a possible return in six months, it makes sense to deal with the factors that turn the flu and SARS we are used to into seasonal diseases.

Why in winter

The fact of the seasonality of colds has been obvious to people since antiquity, but it is not so easy to explain Seasonality of Infectious Diseases. For example, the Roman Lucretius assumed On the Nature of the Universe that "pestilence and plague" are caused by the atoms of disease, which appear when the earth is saturated with moisture. And his compatriot Galen directly attributed to Galen's art of physick outbreaks of various diseases to seasonal characteristics: excessive heat, dryness or cold. Today we know that Lucretius was closer to the truth: it's not about the cold, but about the humidity Absolute humidity modulates influenza survival, transmission, and seasonality of the air.

This was shown by Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature in a laboratory experiment on guinea pigs. Four influenza-infected and four healthy gilts were kept in chambers where the temperature and humidity were changed: the rate of transmission of the virus increased as they decreased. The virus was transmitted best at temperatures of 5 degrees rather than 20 degrees and 30 degrees. At 5 degrees Celsius, the transmission frequency was 100 percent at a relative humidity of 20 and 35 percent; 75 percent at 65 percent relative humidity, but only 25 percent at 50 percent relative humidity; and 0 percent at 80 percent relative humidity.

Several years later, other authors analyzed the Absolute humidity modulates influenza survival, transmission, and seasonality of the same data and corrected the conclusions. They decided to evaluate the effect of absolute humidity, not relative humidity. After recalculation and new experiments, the original conclusion was confirmed, but with the difference that the transmission of the virus is more dependent on humidity than temperature.

The influenza virus was transmitted from mumps to mumps by airborne droplets: when a sick mumps exhales, droplets of water vapor loaded with viral particles enter the air. Once free, the drops gradually settle and evaporate. The faster they evaporate, the slower they settle and the longer the virus hangs in the air. The evaporation rate of droplets depends on humidity - the more steam, the slower it evaporates. Drops settle in the air saturated with moisture faster, "dragging" the virions with them.

And since the humidity drops along with the temperature, the winter time, when it is cold and dry, maximizes the spread of viruses.

The first study evaluated the effect on the transmission of viral particles only at relative humidity - this parameter reflects the proportion of water vapor in relation to its maximum at a given temperature. Moreover, at 20 degrees, this maximum is higher than at 5 degrees.

There is also a second factor here, a purely human one. When people breathe dry air, mucus dries up in the nose, moisturizing the respiratory tract and physically holding back all solid particles, including viral particles. The properties of mucus are associated with special polymeric macromolecules - mucins, which not only impart viscosity to mucus, but also play an important role in the immune response. They form the Barrier function of airway tract epithelium, a special framework that allows you to optimally organize in space the protective proteins that secrete the epithelial cells of the mucous membranes. For example, the glycoprotein lactoferrin Lactoferrin for prevention of common viral infections, which can neutralize lmmunoglobulin concentrations in nasal secretions differ between patients with an IgE ‑ mediated rhinopathy and a non ‑ IgE ‑ mediated rhinopathy, many viruses, including Bovine lactoferrin: involvement of metal saturation and carbohydrate saturation in the inhibition of influenza virus infection influenza virus.

A dry nose leads to several problems at once. First, the epithelium deprived of moisture is more easily damaged, so that it is easier for viral particles to penetrate into cells. Secondly, the spatial organization of mucin is disrupted, lactoferrin and related proteins lose their protective properties, and the body's resistance to the virus decreases.

In addition to humidity, there is another important factor due to which the likelihood of an outbreak of influenza or ARVI in winter is higher than in summer - human behavior. This is supported by the Estimating the impact of school closure on influenza transmission from Sentinel data on the spread of influenza in schools. In autumn and winter, when students spend a lot of time in the classroom, actively communicating with each other, outbreaks of influenza and SARS occur more often than in summer, when students do not attend school and communicate less with each other.

The more people susceptible to the virus gather in one place, the faster and more efficiently the disease spreads.

Annual coincidence

Seasonal Epidemics Emerge Seasonality of SARS ‑ CoV ‑ 2: Will COVID ‑ 19 go away on its own in warmer weather? when a population in which there are many people without immunity (for example, tourists or newborns) encounters a seasonal "helper" of the disease - in the case of influenza, it is low winter humidity.

It looks like this. At the beginning of the epidemic - that is, in the fall - most people do not have immunity to a viral disease, so each patient infects more than one person (R ₀> 1).

Then the proportion of people immune to the virus begins to grow - because those who have been ill develop immunity (or, for example, a vaccine is used). People become infected less and less, and after a while the epidemic reaches its peak (R ₀= 1).

With the arrival of spring, in addition, the air is humidified - so that the conditions for the spread of viral particles are no longer optimal: the protective mucus barrier in most people is restored, the number of vulnerable people drops even more - and the epidemic goes out (R ₀< 1).

COVID- (19 + 1)?

Most viruses that cause respiratory tract infections in humans belong to the Identification of new human coronaviruses in five families: paramyxoviruses, orthomyxoviruses, picornoviruses, adenoviruses, and coronaviruses. And although the flu is caused by orthomyxoviruses, and COVID-19 and some SARS (OC43, HKU1, 229E and NL63) are coronaviruses, all these diseases spread in a similar way.

Coronavirus disease really resembles flu and SARS. The symptoms are very similar, the only difference is in the details: the incubation period is longer, the disease lasts longer, complications occur more often.

	COVID-19	Flu	ARVI
R ₀	5, 7	1, 4–1, 6	1, 4–1, 6
Incubation period (average)	5 days	2 days	1-3 days
Average duration of illness	14 days	7 days	7-10 days
Risk group	People over 65	Pregnant women, children under 5, people over 65, people with chronic diseases	The risk of infection is the same for everyone, complications are extremely rare
The most common complications	Severe bacterial pneumonia	Bacterial pneumonia, sinusitis, otitis media, congestive heart failure	Complications are extremely rare

According to the epidemiologist Vlasov Vasily Viktorovich Vasily Vlasov from the Higher School of Economics, there really is reason to believe that the coronavirus infection will be seasonal.

“Some coronaviruses increase the incidence seasonally (the number of new cases - approx. N + 1) colds, as part of the ARVI totality, says the scientist. - But now you cannot have a well-founded judgment on this matter. The only evidence would be a decrease in the incidence [in the summer], keeping it low and an increase in the incidence in the next season, for example, a year later, and so on for at least two years.”

But there is no reason to believe that it will not be so.

But the current pandemic has lasted less than a year. Because of this, we do not have enough data on which to base assumptions and identify patterns.

Summer hope

Nevertheless, it is still not necessary to expect that the pandemic will go out by summer by itself. Seasonality of SARS ‑ CoV ‑ 2: Will COVID ‑ 19 go away on its own in warmer weather? … The fact is that climatic factors affect the spread of infectious diseases much weaker than herd immunity.

Influenza and ARVI are our old acquaintances, so mankind, at the very least, has learned to defend against them. There are vaccinations against influenza, and the majority of the population has immunity to ARVI. The starting conditions for the onset of an epidemic are unfavorable, therefore, at least some success, these diseases achieve only in favorable conditions - that is, in winter, when dry air plays along with them.

COVID-19 is a new disease, and no one is immune to it. This means that the coronavirus does not need to wait for favorable conditions for spreading - nothing really bothers it.

Relatively speaking, the "coronavirus spring" has not yet arrived, and how long the winter will last is problematic to predict.

“When new pathogens appear, such as the Spanish flu, Hong Kong flu, swine flu and Mexican flu, they cause one or two waves of high incidence,” says Elena Burtseva. - Most often, waves occur either in late spring or in summer, which is not typical for flu. After these one or two waves, people acquire active immunity due to frequent contact with the pathogen. Then this virus gets a chance to become a seasonal pathogen."

However, with coronaviruses, the situation is slightly different, the scientist notes. SARS ‑ CoV came and went in 2002. And cases of MERS ‑ CoV, which was discovered in 2013, still continue to be reported.

“This is due to the fact that the virus can have intermediate hosts and circulate in nature,” says Burtseva. - I will not predict whether COVID-19 can become seasonal. There are seven coronaviruses that affect humans, and four of them are seasonal. Every year we register about 5-7 percent of cases associated with them. These cases are usually mild with no complications. On the other hand, following the example of its two predecessors, COVID-19 may go nowhere."

It is also difficult to make predictions because we do not know how the absolute humidity of the air will affect the spread of COVID-19. However, preliminary data The role of absolute humidity on transmission rates of the COVID-19 outbreak is not in our favor: apparently, in countries with a warm and humid climate (for example, in Singapore), the virus spread no worse than in countries with dry and cold climate (as in some areas of China).

Therefore, the main role in the spread of coronavirus infection, apparently, will play not the climate, but the behavior of people.

As Harvard epidemiologist Mark Lipsitch writes, the only "summer effect" that can be seriously hoped for right now is that the recent findings from Chinese scientists are correct and children are indeed involved in Epidemiology and Transmission of COVID-19 in Shenzhen China: Analysis of 391 cases and 1, 286 of their close contacts in the spread of the disease on an equal basis with adults. Consequently, leaving schools for vacations will have an effect. Because in the case of new diseases, the only way to break the chain of transmission in a vulnerable population is to limit contact between sick and those who are deprived of immunity.

From this point of view, the WHO recommendations seem to be correct: to contain the spread of the virus, self-isolation is recommended for people with cold symptoms Self-isolation if you or someone you live with has symptoms, and for healthy people - social distancing Coronavirus, Social and Physical Distancing and Self-Quarantine …