Will COVID-19 become a seasonal infection?

Infectious diseases causeInfectious diseases external causes - bacteria, viruses, parasites or fungi. Many of them are seasonal - outbreaks occur at the same time of the year. For example, in the Northern Hemisphere, the flu comesGlobal 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 chickenpox outbreaks are more commonRECURRENT OUTBREAKS OF MEASLES, CHICKENPOX AND MUMPS: I. SEASONAL VARIATION IN CONTACT RATES in the spring.

Noncommunicable diseases causeNoncommunicable diseases all other reasons: from genetic problems to trauma - they are not contagious. Such diseases can be massive, but they do not strictly depend on the season. For example, cardiovascular diseases die every yearNoncommunicable diseases 17.9 million people, 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 dependSeasonal infectious disease epidemiology from the weather.

Vitality of the pathogen

The causative agent of cholera - cholera vibrio - is able to survive for monthsEnvironmental reservoirs of Vibrio cholerae in stagnant water, and influenza virus particles, for example, on banknotes, retainSurvival of Influenza Virus on Banknotes infectiousness is only from one to three days. Although after this period, viral particles from banknotes do not disappear anywhere, during this time they receiveMechanisms by Which Ambient Humidity May Affect Viruses in Aerosols the capsid (viral envelope) is 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 virus flu influencesGlobal environmental drivers of influenza temperature and humidity. In countries with a temperate climate, 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 affectedInfluence 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 - vibrio dies faster, and it is less likely to infect someone.

Contagiousness, that is, infectiousness

EvaluatingUnraveling R₀: Considerations for Public Health Applications the rate of spread of the disease, epidemiologists use a metric R₀ Is the average number of people who can catch the disease from one sick person. Measles, for example, is highly contagious: one patient infectsThe basic reproduction number (R0) of measles: a systematic review from 12 to 18 people. Flu is ten times weakerModeling influenza epidemics and pandemics: insights into the future of swine flu (H1N1), his R₀ — 1,4–1,6.

R₀ strongly depends on the behavior of people, which differs from season to season: therefore, some diseases are "seasonal" in this sense. For example, in autumn and winter, schoolchildren spend a lot of timeSeasonal infectious disease epidemiology in the same room, and the number of cases of measles increases dramatically. And in the summer they go on vacation, cease to be in close contact, and outbreaks of this disease are recorded less often.

Elena Burtseva, Head of the Laboratory of Influenza Etiology and Epidemiology at the Institute of Virology of the Gamaleya Research Center for Epidemiology and Epidemiology, noted in a conversation with N + 1 that an increase in the incidence of many ARVI 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 canFactors Influencing the Seasonal Patterns of Infectious Diseases influence outbreaks of disease, - features of the human immune system, depending on the season. For example, with the onset of cold weather we spend less and less time outdoors and wear clothes that cover the body. As a result, less ultraviolet radiation gets on the skin and the synthesis of vitamin D, which plays an importantVITAMIN D FOR TREATMENT AND PREVENTION Of INFECTIOUS DISEASES: A SYSTEMATIC REVIEW OF RANDOMIZED CONTROLLED TRIALS role in protection against bacterial and viral infections. However, there is empirical evidence that people who take this vitamin in pills get sick with the flu at all.Shortcomings of Vitamin D ‑ Based Model Simulations of Seasonal Influenza not less often than those who do not drink vitamins.

Transfer method

Some diseases are transmitted directly, and some are indirectly transmitted. Transmitted directly from the sourceWhat You Need to Know About Infectious Disease flu and ARVI, 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 reproducesTHE ECOLOGY OF AFRICAN SLEEPING SICKNESS in the rainy season, and besides, it livesEpidemiology of human African trypanosomiasis 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 goes for 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 the 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 useSuccessful containment of COVID-19: the WHO-Report on the COVID-19 outbreak in China ARVI or influenza outbreak models for predicting COVID-19 outbreaks.

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 through six months, it makes sense to deal with the factors that turn the flu and ARVI we are used to into seasonal disease.

Why in winter

The fact of the seasonality of colds was obvious to people from antiquity, but to explainSeasonality of Infectious Diseases its not so easy. For example, the Roman Lucretius assumedOn the Nature of the Universethat "pestilence and plague" are caused by the atoms of disease, which appear when the earth is saturated with moisture. And his compatriot Galen directly attributedGalen's art of physick outbreaks of various diseases with 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 humidityAbsolute humidity modulates influenza survival, transmission, and seasonality air.

It was possible to showInfluenza 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 cells 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 analyzedAbsolute humidity modulates influenza survival, transmission, and seasonality the same data also 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 virus depends more 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 assessed the effect on the transmission of viral particles only with relative humidity - this parameter reflects the proportion of water vapor in relation to its maximum at a specific temperature. Moreover, at 20 degrees this maximum is higher than at 5 degrees.

There is also a second factor here, purely human. When people breathe dry air, mucus dries up in the nose, moisturizing the airways and physically holdingThe interaction between respiratory pathogens and mucus all solid, 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 formBarrier 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 lactoferrinLactoferrin for prevention of common viral infectionsthat can neutralizelmmunoglobulin concentrations in nasal secretions differ between patients with an IgE ‑ mediated rhinopathy and a non ‑ IgE ‑ mediated rhinopathy many viruses includingBovine lactoferrin: involvement of metal saturation and carbohydrates in the inhibition of influenza virus infection influenza virus.

A dry nose leads to several problems at once. Firstly, the epithelium deprived of moisture is more easily damaged, so that it is easier for viral particlesThe interaction between respiratory pathogens and mucus get 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 that makes the likelihood of an outbreak of influenza or SARS higher in winter than in summer - human behavior. This is supported by the dataEstimating the impact of school closure on influenza transmission from Sentinel data the spread of influenza in schools. In autumn and winter, when students spend a lot of time in the class, actively communicating with each other, outbreaks flu and SARS occur more often than in the summer, when students do not attend school and communicate less with each other friend.

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

Annual coincidence

Seasonal epidemics ariseSeasonality of SARS ‑ CoV ‑ 2: Will COVID ‑ 19 go away on its own in warmer weather?when a population in which there are many deprived immunity people (for example, tourists or newborns) are faced with a seasonal "helper" of illness - in the case of the flu, 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 (R₀ > 1).

Then the proportion of people who are immune to the virus begins to increase - 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 (R₀ = 1).

With the arrival of spring, in addition, the air is humidified - so that the conditions for the spread of viral particles cease to be optimal: the protective mucus barrier in most people is restored, the number of vulnerable people drops even more - and the epidemic goes out (R₀ < 1).

COVID- (19 + 1)?

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

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

According to the epidemiologistVlasov Vasily Viktorovich Vasily Vlasova 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 on this occasion you cannot have a well-founded judgment. The only evidence would be a decrease in incidence [in summer], keeping it low, and an increase in 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 lasts less than a year. Because of this, we do not have enough data on the basis of which we can make 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 SARS are our old acquaintances, so mankind, at the very least, has learned to defend itself against them. There are vaccinations against influenza, and the majority of the population has immunity to ARVI. The starting conditions for the outbreak of an epidemic are unfavorable, so 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. It means that coronavirus there is no need to wait for favorable conditions for distribution - nothing really bothers him.

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, the Hong Kong flu, the swine flu and the 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 influenza. 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, the situation with coronaviruses 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. - Whether COVID-19 can become seasonal, I will not predict. There are seven coronaviruses that affect humans, and four of them are seasonal. Every year we register about 5-7 percent of cases related to 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 will affect the spread of COVID-19. However, preliminary dataThe role of absolute humidity on transmission rates of the COVID ‑ 19 outbreak not in our favor: apparently, in countries with warm and humid climates (for example, in Singapore), the virus spread no worse than in countries with dry and cold climates (as in some areas China).

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

As Harvard epidemiologist Mark Lipsitch writes, the only "summer effect" on which you can now seriously hope is that the recent results of Chinese scientists are correct, and children really are involvedEpidemiology 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 a par with adults. Consequently, leaving schools for vacations will have an effect. Because in the case of emerging diseases, the only way to break the chain of transmission in a vulnerable population is to limit contact between sick and those who are not immune.

From this perspective, the WHO recommendations seem to be correct: self-isolation is recommended for people with cold symptoms to contain the spread of the virusSelf-isolation if you or someone you live with has symptoms, and for healthy people - social distancingCoronavirus, Social and Physical Distancing and Self-Quarantine.