Opinion: If Covid-19 is a seasonal virus, why is it spreading during the summer?

Understanding seasonality can help us to work out when the pandemic is likely to be over, says Professor Francois Balloux (UCL Genetics Institute).

There is a paradox at the heart of Sars-Cov-2 transmission that has yet to be fully explored. While it’s firmly established that the virus transmits best in winter, in common with most other respiratory viruses, the UK is currently experiencing a summer surge. There have also been major Covid-19 epidemic waves in regions such as South America and India outside winter. Why is this?

From the beginning of the pandemic, one major question was to what extent Covid would display a seasonal behaviour with higher case numbers in winter. Most respiratory viruses in circulation, including the flu and the four endemic coronaviruses contributing to "common colds", display strong seasonal patterns, with most infections happening in winter in each hemisphere.

The mechanisms behind this seasonality are complex and not fully understood. They include the direct effect of climatic variables: enveloped RNA viruses, such as Covid-19, survive best in cold, dry air and under low UV light exposure. There is also a major host behavioural component, with humans tending to spend more time in close contact in poorly ventilated spaces during winter.

Yet, a virus being seasonal does not imply it is unable to transmit at certain times of the year, as long as conditions are otherwise favourable for its spread. To best understand this, we must consider seasonality as just one of four major factors driving transmission. The other three factors are host behaviour, viral evolution and rates of immunisation in the population, provided by prior exposure to the pathogen and/or vaccination.

These four factors will all influence the dynamic of the pandemic. Infectious disease epidemiologists use a mathematical concept called the "R" number to describe the behaviour of epidemics. It represents the mean number of new infections caused by infected individuals over time. When R is greater than 1, every individual infects more than one new host on average, and the number of cases goes up over time. When R is less than 1, case numbers decrease.

Social distancing through behavioural changes, either voluntary or enforced by authorities, will decrease viral transmission. As multiple social distancing interventions are generally enacted 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 behavioural interventions, seasonality or the emergence of more transmissible viral variants.

Viruses will always be under selection to become more transmissible. Entities replicating today will have ancestors that left more descendants. The ability to infect more hosts is key for the evolutionary success of viruses. Increased transmissibility can be achieved through various mechanisms, such as a higher viral load or by bypassing host immunisation, thus allowing variants to exploit a larger susceptible population. The Alpha variant that emerged in late 2020 was intrinsically more transmissible than previous lineages in circulation. The Delta variant, which is responsible for the current surge in the UK, is even more transmissible than Alpha, but also marginally more likely to infect people who are immunised through previous infection or vaccination.

Population immunisation through both 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 "herd immunity threshold", the point at which each infected host infects less than one person on average. This value lies at around 85% for the Delta variant, though, long-term, stable herd immunity against Covid is unlikely to be achieved because immunity will wane over time. Moreover, while current vaccines are remarkably effective at reducing transmission, morbidity and mortality, they do not block 100% of infections.

Population immunisation will continue to increase through vaccination and infection to reach an equilibrium value. The UK is likely getting close to this value already with more than 90% of the adult population having antibodies against Covid. The proportion of the population immunised will constantly be pushed down by waning immunity, new births and the emergence of new viral variants capable of partially bypassing immunity, and pushed up by infection, reinfection and vaccination. Covid will continue to evolve but will eventually reach its maximal transmissibility, which is expected to remain higher in winter.

With the majority of the UK population being vaccinated in the near future and a likely return to pre-pandemic contact rates, Covid is expected to soon reach its epidemic equilibrium. At this stage, three out of the four forces will have been removed. With seasonality obviously remaining, it is expected to start driving the epidemic dynamic, pushing R above 1 in winter and below 1 in summer. At this stage, Covid will join the 200 other seasonal endemic respiratory viruses in circulation globally. In short, we need to wait for the worst of the pandemic to be over before seasonality becomes the factor dictating the transmissibility rate of the virus.

The vast majority of infections in previously immunised people is expected to be fairly benign. We failed to eliminate Covid and are unlikely to do so in the foreseeable future. Although, thanks to effective vaccines and a remarkable viral surveillance infrastructure allowing us to update vaccines whenever needed, those lucky enough to live in rich countries are facing a much-tamed foe, in time likely comparable in its associated morbidity and mortality to the flu, or even the four common cold coronaviruses already in circulation.

The transition of Covid into endemicity is expected to happen progressively with epidemic wavelets of diminishing amplitude settling in the winter. This seasonal endemic state will be attained at a different time in various places. For most of Europe and the US, the transition towards endemicity is already well under way, and epidemic waves linked to massive morbidity and mortality, such as the ones we experienced before, are unlikely to occur in the future.

This article was first published in The Guardian on 16 July 2021.

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