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COVID questions: AC, superspreaders, herd immunity

July 30, 2020

Editor’s note: We will be publishing answers to questions about COVID-19 and the pandemic each week in this COVID questions column. If you have a question, please email it to

Q: How far can the virus travel in HVAC systems, and what’s the risk for infection? For example, my neighbor’s air conditioner blasts an exhaust stream near our bedroom windows. If they were sick, could the virus be transported from their house to ours through an open window?

In the case you describe below, there is virtually no risk of a virus being transported from a neighbor’s house who may have infected occupants.

The portion of a residential air conditioner that sits outside is called the “condensing unit.”  The condensing unit simply takes ambient air and circulates it across a heat exchanger to condense refrigerant that is circulating throughout the air conditioning system.  No air from your neighbor’s house circulates through this unit – only refrigerant and the refrigerant is contained within the system.

– Douglas Reindl, professor, engineering professional development, mechanical engineering; director, Industrial Refrigeration Consortium

Q: I recently came across the term, “superspreader.” What – or who – is a superspreader?

You may have heard statistics like this one: On average, one person infected with SARS-CoV-2 spreads the virus to two other people. Unpacking this average, we find that there is wide variation among individuals. Many infected people do not spread the virus on to others at all. A few infected people can spread the virus to dozens of other people in a single event. These are superspreaders. There have been reports of superspreading events at gatherings like a parties, church choir practices, workplaces, and so forth.

It’s important to know how superspreading fits into the overall transmission of SARS-CoV-2 because it has implications for how we control the spread. You can imagine that if a high proportion of infections result from superspreading, our strategies for interrupting transmission might be different than if superspreading occurred rarely or never. Unfortunately, I don’t think we can predict who might be a superspreader — these events are hard to study since they can’t really be planned for. There’s a great article on superspreading by the science journalist Kai Kupferschmidt that has more information:\

– Thomas Friedrich, Professor, Pathobiological Sciences; Virology Services unit head, Wisconsin National Primate Research Center

Q: How does herd immunity work? Would it work for the coronavirus, and where does a vaccine fit into herd immunity?  

A: Herd immunity is a concept from epidemiology. It definitely applies to the coronavirus. To explain it, I will first introduce one other concept, which is the susceptible, infected, recovered (SIR) model. You can imagine that, during an infectious disease outbreak, we can classify people into three groups.

  • Susceptible people have not been infected yet, and can get infected if they are exposed.
  • Infected people are, well, currently infected, and can spread the infection to others.
  • Recovered people had the disease once, but no longer do. Once they recover, let’s assume they are immune and can no longer be infected, at least for a while. (In these models, people in the “recovered” category may also have died from the disease; either way they can’t be infected again.)

The spread of a virus like SARS-CoV-2 through the population depends (among other factors) on what proportion of the population falls into each group. When SARS-CoV-2 first emerged, essentially the entire human population was susceptible. This means it was pretty easy for the virus to keep finding new susceptible hosts, and so it could spread quickly. This situation still prevails now. This is why social distancing and masking are so important.

But you can imagine that, over time, as more people get infected and then recover, a larger and larger subset of the population will fall into the “recovered” category. Again, let’s assume that recovered people are immune to SARS-CoV-2. The more immune people there are, the fewer susceptible people there are. So if I get infected and everyone I know is recovered, I will not spread the virus to anyone else. The chain of virus transmission stops with me.

At some point, there are enough immune people in the population that the virus has a very hard time sustaining a chain of transmission from one person to another to another, so large-scale outbreaks become highly unlikely. At this point the population has reached herd immunity — enough people in the population are immune that, even if one person gets infected, the chances of a large outbreak affecting lots of people are very low. This is an ideal situation, because even vulnerable people like the very young and the elderly are protected by the immunity of the group.

The proportion of the population that must be immune to achieve herd immunity is probably different for every virus — it could be around 60% of the population, but for some viruses it’s been estimated to be more like 90% or even higher.

We do not know what the number is for SARS-CoV-2, but suppose that we need 70% of the population to be immune to achieve herd immunity. Out of 330 million people in the US, that means we would need to have about 230 million get infected. As of today we’ve had a little more than 4 million known infections in the US. Now, not all infected people get tested. So let’s say that testing only catches about 10% of infected people. That would mean that 40 million people in the US have already been infected. That’s a huge number, but it’s still almost 200 million people short of the number we might need for herd immunity! At the current infection rate of about 60,000 per day in the US, it would take years of living with current pandemic conditions, with all the attendant disease and suffering, to achieve herd immunity without a vaccine.

Vaccines are so important because they allow us to get to the point of herd immunity faster and more safely than we would by just allowing the virus to infect people. Once a safe and effective vaccine becomes available, we can essentially convert people from the susceptible category to the recovered (immune) category without the danger of COVID-19 disease. Once enough people are vaccinated, we will achieve herd immunity.

It’s important to remember that the day a successful vaccine is announced will not be the end of the pandemic. We will need to get vaccine to a lot of people, so that herd immunity is achieved in most communities in the US. That will take time, and considerable money and effort. Also, studies so far suggest that even in places that were very hard hit by the first wave of COVID-19, the population is not close to herd immunity. For example, in NYC it has been estimated that maybe 15-25% of the population is immune to SARS-CoV-2, so they remain susceptible to new outbreaks.

– Thomas Friedrich, Professor, Pathobiological Sciences; Virology Services unit head, Wisconsin National Primate Research Center

See more answers to COVID questions here and here and here and here. Also, visit our COVID-19 impact site.