With no effective treatment against the coronavirus available and a vaccine unlikely to be ready for at least a year, a lot of hope has been placed on the idea that the rise in temperatures over summer will stop the spread of the novel coronavirus.

Fernando Belda, a spokesperson from Spain’s AEMET weather agency, is optimistic about the chances. His team has just found the “first signs of correlation” between cold weather and the spread of the coronavirus in Spain. “We are seeing a pattern, the lower the temperature, the greater the effect,” he explains. But past epidemics and the coronavirus situation in other countries indicate that summer will not be enough to stop the pandemic.

There are a lot more factors that affect the transmission and spread of the new virus, but there is a statistical correlation Cristina Linares, AEMET

Belda and his team of Spanish researchers analyzed the average temperature in each region over a 14-day period and the number of coronavirus cases that were confirmed during this time, for every 100,000 inhabitants. The pattern, according to Belda, is repeated over the 14 days, from the start of the lockdown until now.

Cristina Linares, the co-author of the study, says the results need to be taken with care. “We have to be very cautious, because the humidity and temperature conditions vary a lot between different geographic areas, and of course, there are a lot more factors that affect the transmission and spread of the new virus,” she says. “But there is a statistical correlation.”

Just a few weeks ago, the US National Academies of Sciences, Engineering and Medicine (Nasem) sent a public report to the White House that said summer could not be depended upon to stop the pandemic. The experts warned that countries such as Australia and Iran, which both have weather conditions considered summery, are seeing the rapid spread of the virus. “What’s more, other coronaviruses that cause potentially serious human diseases, such as SARS or MERS, have not shown any seasonal behavior,” added the report, which was led by doctor Harvey Fineberg from Harvard University.

So-called “General Snow,” the name given to the harsh winter climate of Russia that helped stop several invasions, was Russia’s deadliest weapon against Adolf Hitler and Bonaparte Napoleon. Similarly, US President Donald Trump has backed the idea that a General Summer could defeat the novel coronavirus since the beginning of the crisis. At a rally in February, he said: “It looks like by April, in theory, when the weather gets a little warmer, [the coronavirus] miraculously goes away.” But few still believe in “General Summer.”

In a virus with a lipid casing you expect to see a correlation with meteorological variables Carl Heneghan, the director of the Center for Evidence-Based Medicine, Oxford University

There are infections that are closely linked to cold weather, such as the respiratory syncytial virus (RSV), which is the most frequent cause of pneumonia in children. “SARS-CoV-2 has a lipid casing, like the respiratory syncytial virus, which should make it more sensitive to changes in temperature, relative humidity, and ultraviolet radiation,” says epidemiologist Carl Heneghan, the director of the Center for Evidence-Based Medicine, Oxford University. “In a virus with a lipid casing you expect to see a correlation with meteorological variables.” The big question is whether that sensitivity will be enough to stop the pandemic.

According to the report from Nasem, “there is some evidence to suggest that SARS-CoV-2 may transmit less efficiently in environments with higher ambient temperature and humidity.” But it warned: “Given the lack of host immunity globally, this reduction in transmission efficiency may not lead to a significant reduction in disease spread without the concomitant adoption of major public health interventions.” Spanish virologist Margarita del Val, from the Spanish National Research Council (CSIC), agrees: in a pandemic the number of susceptible cases matters more than the meteorological changes.

Epidemiologist Marc Lipsitch, from Harvard University, warned a month ago that even seasonal infections can occur out of season if the virus is new. Classical viruses need favorable conditions, such as winter temperatures, to spread through a population, as many people are already immune after having overcome the disease in previous years. “New viruses have an advantage that is temporary, but important: Few or no individuals in a population are immune,” Lipsitch explained in an article for the US Center of Communicable Diseases (CDC).

The 2009 H1N1 flu pandemic, for example, started in April, eased in the summer, and flared up again in September, all outside of the usual flu season. But Lipsitch, an expert on the current crisis, warned not to expect a summer miracle in the case of the coronavirus. “While we may expect modest declines in the contagiousness of SARS-CoV-2 in warmer, wetter weather [...] it is not reasonable to expect these declines alone to slow transmission enough to make a big dent.”

The epidemiologist Cristina Linares also acknowledges that even the correlation between cold weather and coronavirus cases could be mistaken. “They are preliminary results. Other factors that influence the possible seasonality of the spread must be taken into account, in addition to the environmental conditions. Human activity, containment measures, population density, etc. have a decisive influence,” she explains.

A study done in China at the start of the pandemic suggested that for each 1ºC rise in temperature, the daily number of confirmed cases fell between 36% and 57%

The Spanish team is now working on a more sophisticated analysis, that includes other essential environmental variables, such as humidity, ultraviolet radiation, and air pollution, as well as other factors, including hospital and intensive care admissions, and mortality rates. The objective is to identify “risk areas in real time at the provincial level” to be able to act effectively, according to a statement from the Science Ministry.

A study done in China at the start of the pandemic suggested that for each 1ºC rise in temperature, the daily number of confirmed cases fell between 36% and 57%, as long as the relative humidity hovered around 75%. But the authors of the study – led by Zhijie Zhang from Fudan University in Shanghai – recognized that this association between Covid-19, temperature and humidity was not consistent across different Chinese provinces.

On March 22, the Center for Evidence-Based Medicine at Oxford University analyzed preliminary studies, particularly in China, that pointed to a decisive link between cold, dry weather and the increased spread of the coronavirus. The Oxford team warned that there were methodological errors in the studies and that the papers had not been externally reviewed by other scientists, as is standard practice in international science. For now, there is no reliable sign that “General Summer” can defeat the new coronavirus.

English version by Melissa Kitson.