If the coronavirus is a seasonal virus, why is it spread during the summer?

There is a question related to the current pandemic that millions of people are asking and it has not yet been properly clarified. If Sars-Cov-2 is a seasonal virus, why is it spread during the summer?

Professor Francois Balloux, director of University College London Genetics Institute seeks to answer the question in an interesting column published in The Guardian. Although the virus has been shown to spread best in winter with cold, poorly ventilated places, like most other respiratory viruses, today, the northern hemisphere is experiencing a summer surge. It also happened in the southern hemisphere in December, January and February with large waves of COVID-19 epidemics. Why is it, if the majority of circulating respiratory viruses, including influenza and the four endemic coronaviruses that contribute to “common colds”, show strong seasonal patterns and most infections occur in winter in each hemisphere?

But the scientific answers are more complex than the seasonality of the climate, although these include the direct effect of climate variables: enveloped RNA viruses, like COVID-19, survive better in cold, dry air and with low exposure to ultraviolet light. . There is also an important component of host behavior, as humans tend to spend more time in close contact in poorly ventilated spaces during winter. However, just because a virus is seasonal does not mean that it cannot be transmitted at certain times of the year, as long as conditions are favorable for its spread.

The four factors that spread the virus

To better understand this, we should consider seasonality as one of the four main factors of transmission. The other three factors are host behavior, viral evolution and immunization rates in the population, provided by previous exposure to the pathogen and / or vaccination. All of these four factors will influence the dynamics of the pandemic. Infectious disease epidemiologists use a mathematical concept called number “R” to describe the behavior of epidemics. Represents the average number of new infections caused by infected people over time. When R is greater than 1, each individual infects on average more than one new host and the number of cases increases over time. When R is less than 1, the number of cases decreases.

It is scientifically proven that social distancing through behavior changes, whether voluntary or imposed by authorities, reduces viral transmission. Since multiple social distancing interventions are typically implemented together and in conjunction with other factors, it can be difficult to estimate the effectiveness of each public health intervention in isolation. For example, the effectiveness of school closures is still hotly debated, in part because the impact of this measure remains difficult to disentangle from confounding variables such as other behavioral interventions, seasonality or the emergence of viral variants. more transmissible.

Viruses will always seek to survive and be more transmissible. The ability to infect more hosts is the key to the evolutionary success of viruses. Higher transmissibility can be achieved through several mechanisms, such as a higher viral load or by avoiding host vaccination, allowing the variants to exploit a larger susceptible population. The Alpha variant that emerged in late 2020 was inherently more heritable than previous bloodlines in circulation. The Delta variant, which is responsible for the current rise in the UK, is even more transmissible than Alpha, but is also slightly more likely to infect people immune from a previous infection or vaccination.

Immunization of the population by prior natural infection and vaccination will decrease the R number by reducing the fraction of the host population through which the virus can spread. As the proportion of the population that has been vaccinated and / or previously infected increases, the population approaches the ‘group immunity threshold’, the point at which each infected host infects less than one person on average. . This value is approximately 85% for the Delta variant, although long-term stable herd immunity is unlikely to be achieved. against Covid because immunity will decrease over time. In addition, although current vaccines are remarkably effective in reducing transmission, morbidity and mortality, they do not block 100% of infections.

Immunization of the population will continue to increase through vaccination and infection until it reaches a steady-state value. The UK is probably already approaching this value with over 90% of the adult population having antibodies to Covid. The proportion of the vaccinated population will be constantly reduced by the decrease in immunity, new births and the appearance of new viral variants capable of partially evading immunity and increased by infection, reinfection and vaccination. The Covid will continue to evolve, but will eventually reach its maximum transmissibility, which is expected to remain higher in winter.

With the majority of the population vaccinated in the near future and a likely return to pre-pandemic contact rates, Covid is expected to reach epidemic equilibrium soon. By this point, three of the four forces will have been removed. Seasonality obviously remaining, it should start to drive the dynamics of the epidemic, pushing R above 1 in winter and below 1 in summer. At this stage, Covid to join 200 other seasonal endemic respiratory viruses circulating around the world. In short, we must wait for the worst of the pandemic to pass before seasonality becomes the factor that dictates the rate of transmissibility of the virus.

The vast majority of infections in people previously immunized are expected to be fairly mild. We have not been able to remove Covid and it is unlikely that we will do so in the foreseeable future. Although, thanks to effective vaccines and a remarkable viral surveillance infrastructure that allows us to update vaccines whenever needed, those fortunate enough to live in wealthy countries face a highly domesticated enemy, likely comparable over time in terms of morbidity and mortality associated with influenza. or the four cold coronaviruses that are already in circulation.

The passage of the Covid to endemicity should be done gradually with epidemic waves of decreasing amplitude settling in winter. This seasonal endemic state will be reached at a different time in various places. In much of Europe and the United States, the transition to endemicity is well underway and epidemic waves associated with massive morbidity and mortality, such as those we have experienced before, are unlikely to occur. produce in the future.

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