Understanding how Covid-19 spreads

Why does one virus become a pandemic while others don’t? How and where do we catch them? Following on from their presentation at a recent AHSC online seminar, Professor Wendy Barclay, action medical research chair, virology, at Imperial College London and Professor Frank Kelly, Humphrey Battcock chair in environment and health at Imperial College London, discuss their research into detecting Covid-19 in the environment and how respiratory viruses like influenza and Covid-19 spread between people.

Professor Wendy Barclay, action medical research chair, virology, Imperial College London.

For many years, I’ve worked on another virus that causes pandemics – influenza. Thinking about how flu spreads and looking at what we know about it alongside Covid-19, I’ve come to the conclusion that there are two criteria that a virus has to satisfy in order to emerge as a respiratory virus pandemic.

First, there has to be no pre-existing immunity to protect us. This is the case with flu, as it mutates, meaning our bodies are encountering a new version of the virus each year; it’s also the case with Covid-19, as it came from animals and hadn’t previously infected humans.

Second, the virus needs to be capable of moving from one host to the next with high efficiency.

One of the keys to this is being able to transmit ‘silently’ – that is, from people who don’t seem to be ill. In 2003, another SARS coronavirus emerged that was notorious for its high mortality rate. But despite a lot of concern, this virus didn’t become a pandemic. The difference was that that virus wasn’t transmitted as well in the early stages of infection, when the person infected was pre-symptomatic.

Covid-19, on the other hand, is very much like flu, in the sense that we get a lot of people who are infected but remain asymptomatic for long enough to spread the virus before they realise they have it.

What exactly makes this virus so good at spreading?

As this article in El Pais, entitled ‘The 12 letters that changed the world’, explains, the Covid-19 virus SARS CoV2 has picked up an extra bit of code – the extra 12 letters – that means it is much more successful at spreading and infecting us, getting into our cells fast and evading an important part of our innate immune surveillance response.

So those 12 extra letters, that by chance combined their way into this virus, turned that virus into one that can spread very well from person to person – because the people infected with it shed more virus, because the virus replicates so well in their airway, and because it gets into the next person much better.

Viruses such as Covid-19 and flu mutate all the time, and at the end of last year a new variant of Covid-19 hit the headlines. Thought to have originated in Kent, the “VUI – 202012/01” strain is believed to be more transmissible – though whether it causes more serious illness is still unclear.

Mutations in a virus are random, and often they won’t make a virus any more transmissible or dangerous. However, in the case of VUI – 202012/01, some of the mutations have affected the spike protein – the part of the virus that binds to receptors in human cells and allows it to infect us.

We are currently working on experiments to compare the new variants and previous strains. One of the key questions is whether the vaccines that have so far been approved to immunise us against Covid-19 will still work on the new variant.

The answer to that is that we’re not completely confident at the moment, and it’s very important that we carry out some analysis of that very quickly.

However, there’s good reason to hope that the vaccines we have will still work: only one mutation in the UK variant spike affects the area where the virus spike protein binds to receptors in human cells, whereas people who get vaccinated make antibodies which attack the protein in several different ways.

Professor Wendy Barclay"It would be unlikely that this single mutation would throw off the ability of all the antibodies that are raised against the vaccine to see the virus. But there’s a possibility that the ability of the antibodies to see the virus is compromised to some extent, and that’s what we need to check out."
Where is Covid-19 spreading?


As well as looking at how the virus spreads from person to person, another important question is, “where is transmission actually occurring?”

During the first wave of Covid-19, at the end of March/beginning of April 2020, we knew there was ongoing transmission of Covid-19 happening in hospitals, so it was very important to understand where the source or that infection was.

We did a number of kinds of sampling in hospitals, including air sampling and taking swabs from surfaces. Of all the 218 surfaces we swabbed in the hospital, around half showed evidence of SARS-Covid RNA (ribonucleic acid which is part of the virus’s genetic material).

Perhaps unsurprisingly, the surfaces we found this RNA on most often were the ones that lots of people touch with their hands – such as computer keyboards and the tops of hand sanitiser dispensers.

While we did find some virus in the air, this was of a much lower level than on the surfaces we tested. We weren’t able to culture infectious virus from these samples, so this tells us where the virus has been, rather than where it is infectious at that particular time.

We’ve also been working alongside Professor Ajit Lalvani, who is the director of NIHR Health Protection Research Unit (HPRU) in respiratory infections. He’s been identifying households where individuals have tested positive for Covid-19 and then going into those households and testing other members to look for evidence of transmission.

Professor Wendy Barclay"It’s early on, but we have some preliminary data that show that by the time the team gets to the household, other people are already testing positive for the virus, indicating that household transmission happens very quickly and very efficiently. The data seems to be telling us that the vast majority of transmissions occur in the home."

Just as in the hospital, the surfaces in the home that tested positive most often were the ones that many people in the household touch often – such as the kettle handle and the control buttons on the microwave.

This research also looked at the role hands in particular might play in the spread of Covid-19. We’ve found that transmission is much more likely in households where hands that were tested were shown to have Covid-19 RNA. This underscores the message, that keeping clean hands, and therefore surfaces in the home, is very important.

In our experience, it’s very hard to find Covid-19 in the air, even in household settings where we know transmission is high. But we do frequently find it on surfaces and hands in places where transmission is occurring.

We can’t, however, rule out that there is significant airborne transmission, and I think we have to bear in mind that people may be breathing out infectious virus early on during the course of infection in the pre-symptomatic phase and that may be most of where this transmission is occurring. What we’re hoping to do going forward is to focus on this early time to discover how infectious a person is during this pre-symptomatic period.


Professor Frank Kelly, Humphrey Battcock chair in environment and health at Imperial College London.

Professor Frank Kelly

In my work, I’ve looked at air pollution and Covid-19, and the possible links that may exist between these important topics. The pandemic has seen an increase in media and political attention in the issue of air pollution, and there are two main reasons for this. The first is that it’s been reported that recorded cases of Covid-19 have been more prevalent in areas of high air pollution; second, Covid-19-induced lockdowns have led to a noticeable decrease in air pollution in urban areas.

We know that air pollution is a really serious health issue. According to the WHO, some 91 per cent of the world’s population is exposed to too much air pollution and seven million premature deaths are now linked to exposure to air pollution. It seems pretty clear that some of the health issues linked with air pollution – such as respiratory and cardiovascular disease – could make people more vulnerable to the effects of Covid-19. But we were also interested in finding out whether air pollution can actually play a role in spreading the virus among the population.

This idea first emerged in April 2020, when research linked pollution in northern Italy with the Covid-19 pandemic that hit that area pretty badly. Local researchers reported that they had found the Covid-19 virus on PM2.5 (also known as fine particulate matter), that comes from a range of sources including vehicle exhausts.

This led to a lot of interest in the UK about a possible link between air pollution and Covid-19 spread, and we were asked by the environment agency to undertake a study across the UK looking for the presence of the virus on pollution particles.

We examined lots of samples, all of which we found to be negative, meaning that we found no evidence that the spread of the virus was linked to particular pollution in the UK.

The next issue was a government briefing back in May from the transport secretary, Grant Shapps, who urged the public to avoid using public transport if possible when we were coming back out of lockdown, and to travel in their own cars if they could. This advice was issued to reduce overcrowding, and it led to a dramatic fall in the use of public transport. Following this, Transport for London approached us to undertake some analysis, which we gladly did.

We were able to take surface and air samples from several TFL services. Again, all the samples proved to be negative, indicating that that the underground, trains and buses were all safe to use.

Another interesting aspect of the pandemic in relation to the environment, is how restrictions associated with it have led to a reduction in vehicle use and, as a consequence, a decrease in levels of air pollution.

In urban areas, we saw a significant fall in air pollution levels (though we did not see the same level of decrease in suburban areas, as people there carried on using their cars).

As we came out of lockdown around May 2020, air pollution increased again – although because there are still restrictions, we haven’t quite yet returned to pre-lockdown pollution concentrations.

So, while there’s no real physical evidence to link Covid-19 with air pollution, it does pose an interesting question regarding a possible ‘green recovery’: as we get ‘back to normal’, will we all choose to get back in our cars, leading once again to congestion in our city centres – or will we change the way we travel, so there is less, and cleaner, traffic on our roads. Post pandemic, it will be interesting to look back on what decisions society made.

Professor Frank Kelly
"While it seems unlikely that air pollution increases the spread of Covid-19, we know that improved air quality is an important measure in helping us to reduce the burden placed on people’s health as well as on our healthcare systems."


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