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Vismita: Hello and welcome to Science in 5. I'm Vismita Gupta-Smith. And this is WHO's conversations in science. This is the series where WHO experts will explain the science related to COVID-19, so you can protect yourself and your loved ones. Today's expert is Dr Maria Van Kerkhove. She's the technical lead for COVID-19. She's also an infectious disease epidemiologist. Welcome, Maria.

Maria: Hi Vismita, thanks for having me.

Vismita: So Maria, we're now in the seventh month since WHO declared this a Public Health Emergency of International Concern and new evidence is coming in every day. So, tell us how this virus infects the human body.

Maria: What is really important that we understand about transmission is how this virus transmits, meaning how does this virus move from one individual to another? When the virus transmits, meaning when does somebody who is infected with this virus transmit to others? And where is transmission taking place? What is the setting in which the virus is spreading? So, what we know about how it transmits. So, this is a respiratory pathogen, a respiratory virus, and most people who are infected with the SARS-CoV-2 virus, tend to have these respiratory symptoms. And the virus can be spread by droplets, or these little particles of liquid that come out of your nose and your mouth when you talk, when you cough, when you sing, when you're in close proximity to somebody else. These particles, these droplets can be various sizes. They could be larger or they could be smaller. The larger droplets tend to drop more quickly and the smaller droplets can remain suspended in the air for a little bit longer. But this virus transmits when you're in close contact with somebody else and when an infected person gets those droplets into somebody else's eyes, nose, or mouth. The other way in which this virus can transmit is through contaminated surfaces. So, if somebody is infected and they have the virus in these droplets, it can fall onto different surfaces. And, if someone touches those surfaces and then touches their eyes, nose, and mouth, they can infect themselves if they haven't washed their hands. But the good news is that if you wash your hands, if you keep physically distanced from people,you could prevent that from happening.

Vismita: So Maria, tell us now about when and where, in which settings this virus is transmitting.

Maria: Now, when a virus transmits from an infected person to another is really important.What we know about this virus is that people can transmit the virus when they don't have symptoms. So they can be infected with the virus and not yet have developed symptoms. Or they can pass the virus when they do have symptoms. It appears that most people are infectious, most infectious at, or around the time that they develop symptoms, which includes a few days before they actually get sick or actually start to feel unwell. And then where the virus transmits. So, the setting in which the virus spreads is a combination of the intensity of the exposure somebody has, the duration of exposure they may have to an infected individual, and then the location of where that's taking place. So for example, we know that in crowded settings, when people are in close proximity to one another, they spend long periods of time, they're in a room that's crowded, may not have good ventilation. That is a good example of where transmission can take place readily. 

Vismita: Maria, we are also learning new information about how our body's immune system responds to this virus. Tell us what we know today about our immunity. Does it last, for instance?

Maria: So, when an individual is infected with the SARS-CoV-2 virus, they develop an immune response. It usually takes a week or two, and sometimes maybe longer for the body to develop what we call antibodies and these protect against reinfection. What we don't have a complete picture of yet is how strong that protection is, depending on the type of infection you have: If you have asymptomatic infection, meaning you don't have any symptoms; if you have mild disease; if you have severe disease. It's not completely clear how strong the protection is and how long that protection lasts. But we do expect people who are infected with this virus to develop an immune response.

Vismita: Maria, we're also hearing about situations where  people are reported to have been reinfected. What do we know about that?

Maria: Yes, we are hearing reports that some people may have been reinfected with the SARS-CoV-2 virus. But it's important to put this into context out of the more than 23 million cases that have occurred worldwide. We need to understand how often this is happening at a population level. When somebody develops an immune response, they have these antibodies that last for a certain amount of time, and we really need to understand how long those antibodies last, when those antibodies are present, that will protect against reinfection. But if those antibodies wane over time, if they reduce over time, it may be possible that somebody could be reinfected again. 

Vismita: And as new evidence comes in and we learn more, we will keep you informed about the science on this issue. Maria, tell us about the mutations that this virus is going through. We hear about different strains. Is this something we are worried about?

Maria: All viruses change or you can call it mutate. Mutate sounds like a scarier word than change. But there are natural changes and natural mutations that happen with the virus as it circulates through a population. There are some changes that have been occurring and we are following all of the changes of the virus, through a global network of laboratory scientists and virologists, who are helping us to evaluate all of the full genome sequences that are becoming available. There are tens of thousands of full genome sequencesthat are being provided by countries so that we can look at these changes.And we're working with a group of scientists who are helping us to look at each of those changes to determine which ones are really important as we understand how the virus behave.

Vismita: There you have it. That was Dr Maria Van Kerkhove explaining the science of how SARS-CoV-2 virus infects our body and how we react to this virus. Remember, we'll keep bringing you the science as we learn more. Until then, stay safe and stick with science.

Speaker key

Vismita   Vismita Gupta-Smith

Maria   Dr Maria Van Kerkhove