For each person infected, COVID-19 spreads to about 6 more

INDIANAPOLIS (WISH) – The new surge of COVID-19 cases has seen scientists looking again at just how quickly the coronavirus can spread. 

They’re looking at what’s called “the R naught number,” which is the scientific ratio of how many people one person with the virus can infect. 

Indiana State Department of Health reported Tuesday that 5,447 additional Hoosiers have tested positive for COVID-19, for a total of 392,663. In addition, the state recorded 124 more deaths, putting the Indiana death toll from the coronavirus at 6,109.

News 8 spoke with a public health expert, Dr. Richard Pan, who says the current coronavirus R naught is much too high.     

Gillis: We are talking about probability theory and the role it has played in terms of COVID-19 and the pandemic. It’s certainly played a very important role and has helped us anticipate the spread, the public health threat…times when we are most vulnerable and we need to scale back and then when we can open up. In simplest terms, what is probability theory, how has it helped us during this pandemic and is it a reliable lens through which we can make decisions or have been making decisions in terms of COVID-19?

Pan: With COVID-19 or any other threat to our health we think about risk and what is our level of risk. So, when it comes to COVID-19 what we want to think about is what are the things we can do to lower our risk of both exposure and therefore getting the disease as well as transmitting it to other people because it is a contagious virus. We don’t want to either get it or give it to other people

When we think about things that we need to do, we have to realize that in order to reduce the risk of getting COVID-19…what we want to do is first of all reduce the amount of virus there is in the community overall. So, that’s about not transmitting it to other people. But then there are also things we can do to reduce our own risk. For example, wearing a mask. That’s been shown to be about 45 percent effective in reducing transmission of the virus. Keeping 6 feet away from people. Again, reduces that transmission. Being outdoors versus indoors or having enough ventilation reduces that risk. 

So, while no one thing in particular will eliminate that risk if you do several of them together then the probability of you getting the virus can shrink dramatically and, in addition, the probability that you can transmit it to other people will decrease. The point is…if everyone does that collectively…so if each one of us decides for ourselves and everyone does it together then we can actually even eliminate the virus in our community and that’s happened in several other countries. 

Gillis: Circling back to things that we can and cannot do to spread the virus…we have this term in probability theory that’s called the R naught. Can you explain what that is and how it relates to COVID-19? And what goes into that number? What are things that prevent that number from going up? Or from going down or staying the same. 

Pan: An R naught is a technical way of describing the number of people an infected person is likely to infect around them. And certainly lots of factors go into that, but it’s sort of an indicator of how infectious the disease is. For example, measles–which is one of the most infectious viruses around–has an R naught around 18. COVID-19 has an R naught of about 5.7–approximately 6. 

That means that if we did nothing to prevent the spread a person with COVID-19 is likely to infect approximately 6 people on average or 5.7…but let’s round it up to 6. But if, for example, you wear a mask, you could reduce that by almost half. So that means now only 1 person would only affect 3 people and if they kept their distance then it would reduce it even further. 

And I would also point out that if you look at the R naught number for a group of people or a population…as long as the R naught is greater than 1 that means the virus is spreading through the community and it’s growing. If you have an R naught for a group of people or population of less than 1 that means the amount of virus is slowly shrinking in the population. 

From a public health standpoint when we’re looking at groups of people or a community we would like to do things to get that R naught below 1 because that means that the virus is slowly decreasing in terms of its spread within the community. So, R naught is a very useful term to help us figure out what are the things we need to do to stop or slow the spread of a virus whether we are looking at it in terms of a community or we can also think about it in terms of individuals and what are the things you can do to try to reduce the spread let’s say in your household or workplace and your likelihood of getting it because someone else has the virus. 

Gillis: So, the R naught is approximately 6 if we were doing nothing to prevent the transmission but there are these modifiable and non-modifiable factors that we can take into consideration. So, the modifiable are what you were saying in terms of wearing our masks and doing social distancing and staying out of places we shouldn’t be, staying away from large gatherings. But in terms of things that we can’t control. Let’s say, for example, now the weather is changing. We have a colder environment outdoors. We have a drier environment indoors. How might this affect the number, in your opinion, moving forward taking into consideration the things we don’t have control over? 

Pan: Keep in mind that the R naught is an average. What happens is someone is infected and if they are in close quarters with a whole bunch of other people then the R naught for that person can get very large. So, if you happen to be in a very large household with ten other people and you’re all very close to each other then the likelihood of infecting people is probably not just going to be 6, but it could possibly be 10. 

So, what happens when the weather gets colder outside? Well, what that does is changes our behavior. And certainly the number of people we might infect–or the likelihood of you getting infected–will increase if it causes people’s behavior to change in a way that is more likely to make you exposed

Think about if it’s winter…it’s cold outside so you’re more likely to be indoors. Indoors is probably less likely to have good circulation and you’re more likely to be with other people and more people. So, it’s more likely that someone in your group in whatever indoor space you’re talking about–your house, your apartment, a workplace, restaurant or bar–which hopefully you’re not visiting right now because of COVID…But if you do go to a place…maybe going shopping…how many people are together and how close are they together?

When you have things like colder weather or let’s say it’s raining outside…what do we all do? We go indoors and if we’re feeling like we want to connect with other people sometimes what happens is we’re getting closer to people and it’s more likely the infection will spread. So, when we’re thinking about COVID-19 what we really want to think about are chains of transmission. How many people does that one infected person infect? That’s that R naught number–which is an average. And then if those people get infected, how many people do those infect?

So, you might have an R naught of 6 for COVID-19. But then you have to think about that 1 person who infects 6 people. Each of those people affect an average of 6 so 6 times 6…that’s 36 and then imagine they each infect another 6 people. You can see if you do the multiplication that number grows very fast. If we can have people change their behavior in a way that reduces the number of people they infect then you can greatly reduce–even if only one of those initial 6 reduces–that still reduces the number by a dramatic amount and if we all work together we can actually prevent this virus from spreading.

The R naught isn’t for any particular person. It’s really a statistical term. So, it doesn’t really apply to individuals. But the concept…if we all collectively do the things we need to do to stop the spread we could actually in the whole population cause the virus to go away. And that’s why I said that if we in the community can get the R naught under 1 that means the virus is slowly going away.

Gillis: Yeah. It’s interesting that it’s an average. For some individuals or communities it may be higher, but for others it’s much lower and we want to get to that point where it’s less than 1.

News 8’s medical reporter, Dr. Mary Elizabeth Gillis, D.Ed., is a classically trained medical physiologist and biobehavioral research scientist. She has been a health, medical and science reporter for over 5 years. Her work has been featured in national media outlets. You can follow her on Instagram @reportergillis and Facebook @DrMaryGillis.