The risk of coronavirus spreading on public transport has remained substantially low through the pandemic, several international studies have shown.
Safety measures imposed on public transport around the world since COVID-19 hit have made them "the safest places on earth", Dr Julian Tang, a professor of respiratory sciences at Leicester University, told Sky News.
He said if people took the same precautions in other high-risk areas such as crowded streets and pubs, the number of cases would reduce there.
The latest data from France shows only 1.2% of the country's 2,830 coronavirus clusters - three or more cases from one place or event in seven days - recorded between 1 May and 28 September occurred on any type of transport (planes, boats and trains).
Two-thirds of them were recorded as being transmitted at businesses, school and university environments, public and private events and health centres.
Hong Kong, one of the most densely populated places in the world and where public transport is heavily relied on, has recorded an overall infection rate of 68 cases per 100,000, far less than Western countries such as the US which has 2,198 per 100,000 and Spain which has 1,602.
Although there is no expert consensus, some researchers suggest the widespread use of masks could be one of the main reasons for the low transmission rates in Hong Kong.
Dr Tang said: "In Asia, there was already a culture of being very vigilant on public transport, so they were using masks in Singapore and Hong Kong as soon as they learned of the virus."
In Japan, the strategy to contain the virus did not rely on mass testing or national lockdowns, but on urging people to stay away from the "three Cs" - closed spaces, crowds and close-contact settings in which people are talking face-to-face.
Japan's approach was based on finding the places with the most outbreaks and determining their common characteristics, Science Magazine reported.
Gyms, pubs, music clubs and karaoke bars had the most outbreaks and all went against the three Cs.
Public transport also had those characteristics, but no clusters have been traced back to the country's busy rail network.
"In Japan, you're not allowed to talk on the subway, so despite the Tokyo Metro being one of the busiest in the world, they managed to stay safe as they were also wearing masks," Dr Tang said.
Public transport expert Mohamed Mezghani, secretary general of the International Association of Public Transport (UITP), told Sky News: "Public transport was stigmatised at the beginning of the pandemic and as a result rail stations, trains and buses were disinfected regularly from early on.
"The wearing of face coverings, in pretty much all countries, was also made mandatory early on."
Droplets and aerosols transmission
COVID-19 is often transmitted from an infected person through droplets generated when they talk, cough, sneeze or exhale.
Some of these convert into aerosol particles, which are lighter than droplets and can be spread further and remain in the air for longer - and anyone who inhales them can become infected.
Several studies have concluded that the transmission of droplets and aerosols can be prevented, or at least limited, by using a face covering, even in more confined spaces such as public transport.
There is also less interaction between people on public transport, so the relative lack of speaking, shouting and laughing reduces transmission.
Eating and drinking is banned on many transport systems around the world and happens very little where it is not banned, meaning those aerosols from eating and drinking are not spreading.
People also generally stay on public transport for less time than if they are in the office or at a restaurant, again minimising exposure.
Social distancing is also key, with many governments reducing public transport capacity during the pandemic.
Ventilation also plays a major role in preventing aerosols remaining in spaces.
"Transport systems increased their ventilation so they are actually much safer than other smaller spaces," Mr Mezghani said.
"Ventilation isn't done through windows, you don't necessarily feel it as it's done mechanically.
"Long-distance trains actually have some of the best ventilation, the French high-speed trains renew air every 2.5 minutes, others are every five minutes."
Surface transmission
Some of the droplets from an infected person are too heavy to remain in the air so fall on floors and surfaces.
Touching those surfaces and then touching your eyes, mouth or nose increases the risk of transmitting the virus - and we touch our face 20 times an hour, on average.
That is why cleaning hands and surfaces is so important and is something public transport systems started increasing straight away.
Transport for London (TfL) now uses a cleaning substance similar to the ones used in hospitals and installed 1,000 hand sanitiser points across its network.
Network Rail has added a warning system on its app to notify passengers if a station is busy, trains and "high-touch" areas in stations are being cleaned more frequently and extra train carriages are being provided where possible to facilitate social distancing, while hand sanitiser is provided in stations.
But, "the effectiveness of hand hygiene is increased when combined with other measures, such as face masks," a SAGE paper found.
Public transport usage remains low
Despite the evidence from several countries of few infections connected to public transport, this might be due to a lack of data and resources.
It is easier to detect clusters in places that already have the infrastructure, such as hospitals or schools, but it is harder to identify them from public transport.
:: Subscribe to the Daily podcast on Apple Podcasts, Google Podcasts, Spotify, Spreaker
Dr Tang added: "If people follow precautions everywhere, it can be effective - as seen across Asia and then Europe - but if people don't continue to follow them then there's a risk.
"It's not just the public transport company or government's responsibility, it's a social contract by the public to adhere to rules to keep people safe."
But, the low number of reported transmissions from public transport might also be affected by a drop in use.
Private vehicle usage is back to pre-pandemic levels, while trains and buses are still down by 60-40%.
In London, Birmingham, Madrid and New York, Apple found the number of people searching for directions on public transport through Apple Maps was lower than at the beginning of the year.
It appears the stigma against public transport is still there and risks negating any positive impact on climate change not using cars had during peak lockdown.
© Sky News 2020
