Focus: How to stop the spread of coronavirus. Video / AP / Mark Mitchell
Coronavirus cases have fallen dramatically in recent weeks, particularly in London, where Public Health England modelling suggests fewer than 24 people a day are now picking up the virus.
But can the reduction be because of lockdown measures and social distancing? Or might the onset of summer and the warmer, brighter weather, be helping to suppress the virus?
Here is what we know about the effect of climate, weather, sunshine and Vitamin D on coronavirus.
Latitude
Latest research suggests that coronavirus is following a very specific path around the globe, leaving some countries unscathed, while having a disproportionately devastating impact elsewhere.The University of Maryland found that most cases fall along a narrow east-west corridor of 30 and 50 degrees of latitude, which includes northern Italy, the Pacific northwest, Japan, Iran, South Korea, France, Spain and Germany. All have similar climates.
In contrast, areas that were expected to be hardest hit because of geographical proximity and travel connections to the Chinese outbreak - such as Southeast Asia - have had low infections and deaths compared to those in the "coronavirus belt".
None of the temperatures in badly affected cities dipped below 0C during the height of the epidemic, which may suggest a threshold beyond which the virus cannot survive.
The University of Oxford also recently conducted a review into whether climate conditions were playing a role in the transmission of coronavirus, and found that cold and dry conditions appeared to boost its spread. It found that while the global death rate was 0.2 per cent, in the northern hemisphere it was 0.3 per cent and even discovered a gradient relationship in Italy with the south being less affected than the north.
Analysis of the previous Sars outbreak in Hong Kong has shown that the number of daily cases was higher on days where the weather was cooler and research has shown that viruses are able to live far longer on surfaces when the weather is cold.
Warmer weather also leads to fewer people huddled together indoors, so the virus has less chance to jump from person to person.
Vitamin D
There is growing evidence that vitamin D helps protect against coronavirus and that people with chronically low levels may be at greater risk.
Researchers from the Anglia Ruskin University compared the numbers of coronavirus cases to the average levels of vitamin D for 20 European countries and found a significant correlation.
Italy and Spain had high mortality rates, and scientists found both countries have lower than average vitamin D levels.
This is partly because people in southern Europe, particularly the elderly, avoid strong sun, and their darker skin pigmentation also reduces the body's ability to produce natural vitamin D.
In contrast, the highest average levels of vitamin D are found in northern Europe, because of a high consumption of cod liver oil and vitamin D supplements, and possibly less sun avoidance.
Scandinavian nations are among the countries with the lowest number of Covid-19 cases and mortality rates per head of population in Europe.
Is Vitamin D the weapon against Covid-19 we've been looking for? Photo / Getty Images
Dr Lee Smith, reader in physical activity and public health at Anglia Ruskin University, said: "Vitamin D has been shown to protect against acute respiratory infections, and older adults - the group most deficient in vitamin D - are also the ones most seriously affected by Covid-19."
A previous study found that 75 per cent of people in institutions, such as hospitals and care homes, were severely deficient in vitamin D.
"Vitamin D modulates the response of white blood cells, preventing them from releasing too many inflammatory cells, which may stop the body overreacting to the virus."
A study by Trinity College Dublin and the University of Liverpool published this week also shows that vitamin D appears to help reduce serious complications in coronavirus patients.
Experts have also pointed out that black and minority ethnic individuals living in northerly latitudes are often deficient in vitamin D, which may partly explain the higher death rates in these groups.
During the Spanish flu pandemic of 1918, doctors noticed that patients who were nursed outside appeared to fare better.
Sunlight is known to be germicidal and evidence is growing that it can kill viruses as well. Viruses tend to survive better in cold weather because they have a fatty protective coating that degrades when it is warm.
Britain has had the sunniest April on record, which has coincided with a substantial fall in new cases.The UK had an average of 224.5 hours of sunshine last month, compared with 211.9 hours in April 2015, the previous highest.
Keith Neal, emeritus professor in the epidemiology of infectious diseases at the University of Nottingham, said: "Sunlight includes ultraviolet radiation. This damages DNA and RNA.
"I have not seen any work on how quickly this affects Covid-19 but viruses left on surfaces outside will dry out and be damaged by UV light in sunlight."
Researcher Dr Richard Hobday, author of The Healing Sun: Sunlight and Health in the 21st Century, said that during World War I, military surgeons routinely used sunlight to heal infected wounds.
"Public health advice in the 1918 pandemic was sleep with your bedroom windows open and get out in the fresh air and sunshine.
"Hospital patients who were nursed outdoors in tents and put in the sun seem to have recovered far better than those indoors."
This week, giving evidence at the science and technology select committee, Professor Alan Penn, the chief scientific adviser at the Ministry of Housing, Communities and Local Government said being outdoors will help prevent people from contracting the virus.
"The science suggests that being outside in sunlight, with good ventilation, are highly protective against transmission of the virus."
VentilationOpen-air therapy was once a popular treatment for deadly respiratory conditions such as tuberculosis, and patients were regularly put outside to breathe fresh air until antibiotics became standard in the 50s.
As early as the 60s, the Ministry of Defence had shown that outdoor air is a natural disinfectant, able to kill the flu virus and other harmful germs.
Hospital wards were once "cross-ventilated" with large windows to allow fresh air to move freely, but modern hospitals mostly have closed systems and depend on air conditioning for their air supply.
In March, scientists from the National Centre for Infectious Diseases in Singapore found that coronavirus could spread around buildings via air conditioning systems or even on a draught, and discovered traces of the virus in a hospital air duct.
Neal said fresh air quickly dispersed any droplets of coronavirus that were in the atmosphere.
"Talking and coughing can produce droplets and aerosols. Droplets, which are larger than aerosols, carry more virus but fall rapidly to the ground under gravity," he said.
"Aerosols are smaller and can drift further but also dry out quickly because they lose water content as they have a high surface area to volume unlike droplets.
"They are also rapidly dispersed with air movements and we rarely have no air movements outside.
