Will Covid-19 become seasonal?

Human alpha and beta coronaviruses and influenza peak in winter months whereas many other respiratory viral pathogens do not.

Surges in incidence of these infections are thought to be due in part to environmental effects on viral stability and transmission as well as host behavior (eg, clustering indoors) and changes in immunity level over time. More specifically, winter months are generally associated with decreased temperatures, decreased absolute humidity, and decreased indoor relative humidity (where we spend most of our time interacting).

Cool, dry environments have been associated with increased influenza and coronavirus stability and transmissivity. This is thought to be due to changes in essential viral outer proteins and lipids, as well as droplet matrices during droplet, and fomite transmission. For example, environments with lower relative humidity may lead to droplet evaporation and smaller droplet sizes. This affects how far virus-containing droplets travel through the air and where they deposit in the airways.

Laboratory experiments have demonstrated that controlling temperature and humidity affects the viability of coronavirus and influenza. SARS CoV-1 was found to have longer viability at temperatures typical to air-conditioned environments (22–25°C) with relative humidity of 0–50% compared with higher temperatures (>38°C) and higher relative humidity (>95%).

SARS-CoV-2 has been found to have similar stability under experimental conditions. One study found that the viability of SARS-CoV-2 decreased at higher temperatures.

Beyond the impact temperature and humidity have on viral stability, researchers have also investigated the impact they have on transmission. Transmission was found to be highly efficient at ambient temperatures of 5°C, variable at 20°C, and inefficient at 30°C.

Lower relative humidity (20%–35%) was also found to be more favorable for spread of the virus.

Because SARS-CoV-2 is an emerging virus, we do not have the long term data to determine whether COVID-19, the disease caused by SARS-CoV-2, cycles seasonally. However, recent research, still preprint and not yet peer-reviewed, has examined ecological associations between transmission patterns and climate.

In one study, researchers found that before March 22, 2020, 90% of SARS-CoV-2 global transmissions occurred within areas with temperatures ranging from 3°C to 17°C and absolute humidity ranges between 4 and 9 g/m³ daily.

Similarly, Sajadi et al found that areas across the globe with significant community spread (defined as ≥10 reported deaths by March 10, 2020) remained in a distribution approximately in the range of 30–50° north latitude with average temperatures of 5–11°C and low absolute humidity (4–7g/m³).

In addition, Notari estimated that among countries with at least 30 COVID-19 cases and 12 days of data before April 1, 2020, the maximal transmission peak occurred at 7.7°C .

A study investigating 30 Chinese provinces found a 1°C increase in average temperature was associated with a decrease in daily confirmed cases by 36%–57% when average relative humidity ranged from 6% to 85.5%. Furthermore, a 1% increase in average relative humidity led to a decrease in daily confirmed cases by 11%–22% when average temperature was between 5.04–8.2°C.

Athough these findings are not consistent throughout China, their conclusions that higher viral spread is observed in areas with moderate temperatures and lower humidity are similar. However, the evidence is still developing and is somewhat inconsistent. In fact, recent research from a public bath centre in Huai’an, Jiangsu Province, China found that a bathhouse with high temperature and humidity was the source of a cluster of COVID-19 cases.

Two primary factors that potentially confound the relationship between environment and transmission include travel and behavioral patterns, which are also driven by public health policy, testing capacity, quality of healthcare, and per capita income. Furthermore, rapid viral spread is not solely limited to areas within ranges of 3–17°C temperature and 4–9 g/m³ humidity. For example, COVID-19 has already spread very rapidly throughout Iran and Louisiana.

As to whether COVID-19 will enter into regular circulation like other human coronaviruses and influenza, this will depend largely on the duration of immunity to the virus, which remains unknown. One study predicts that if duration of immunity to SARS-CoV-2 mimics other related coronaviruses, recurrent outbreaks are likely to occur.