Balmy days are coming, with temperatures forecast to reach 20C in some regions. The warm weather will bring welcome respite to lockdown Britain - and put pressure on authorities trying to control crowds and gatherings. However, scientists also believe warm weather could bring new insights into the virus by showing whether it reacts to the onset of spring. Flu epidemics tend to die out as winter ends; could sunshine, similarly, affect the behaviour of the coronavirus and its spread? It is a key question, and epidemiologists will be watching for changes very closely. Initial studies of other coronaviruses - the common varieties that cause colds in the UK - do suggest a seasonal pattern, with peaks occurring during winter and disappearing in spring. Intriguingly, these peaks tend to coincide with flu outbreaks. By contrast, only small amounts of coronavirus appear to be transmitted in the summer. A key study of the common coronaviruses - HCoV-NL63, HCoV-OC43 and HCoV-229E - was published last week by scientists at University College London. By analysing samples collected several years ago they found high rates of coronavirus infections in February, while in summer they were very low. Other studies have also shown coronaviruses are seasonal in behaviour in temperate climates. The study’s lead author, Rob Aldridge, sounded a note of caution, however. “We could see continued but lower levels of coronavirus transmission in summer but this may reverse in the winter if there is still a large susceptible population at that point,” he said. “And given this is a novel virus, we don’t know if a seasonal pattern will hold over the summer given high levels of susceptibility in the population. For this reason, it is crucial that we all act now to follow current health advice.” This point is backed stressed by other scientists, who warn that the Covid-19 virus is a completely new infectious agent and so there has been no chance for populations to build up any immunity. As a result, it is likely to continue to spread at current rates despite the onset of summer. “I am sure seasonal variations in the virus’s behaviour will play a role in its spread,” said virologist Michael Skinner at Imperial College London. “But compared with the effect we are having with social distancing, it will be a very minor influence. It may produce some marginal effects but these will not be a substitute for self-isolation.” Ben Neuman of Reading University was more emphatic. “This virus started in near-freezing conditions in China, and is rapidly growing both in Iceland and on the equator in Brazil and Ecuador. As winter turned to spring, the virus growth has accelerated worldwide. This is not War of the Worlds, and there is no deus ex machina to reach out of the clouds and put this right. We have to beat the virus ourselves.” The arrival of spring does not only affect the behaviour of a virus, however. It also produces changes in the human immune system, other researchers point out. “Our immune system displays a daily rhythm, but what is less known is how this varies from season to season,” said immunologist Natalie Riddell at Surrey University. To find out, Riddell and other researchers at Surrey and Columbia Universities have been studying immune changes in humans at different seasons and different times of day. Biological samples were taken from volunteers at the winter and summer solstices and the spring and autumn equinoxes. Initial findings suggest a subset of white blood cells that play a key role in the immune system appear to be elevated at certain times of day, indicating that the system responds differently at varying times. For example, B cells that produce antibodies were found to be elevated at night. However, the impact of seasons on cell rhythms is still under investigation, added the study’s leader, Micaela Martinez of Columbia University. Results would be of considerable importance, she added. “Knowing the vulnerabilities of our body to diseases and viruses across the year could inform the timing of vaccination campaigns that will help us eradicate infections.”