July 1, 2020 at 6:08 pm PDT | by Gabriel Rotello
How to stoke an epidemic

A pedestrian walks by a closed sign on the door of a restaurant on March 17, 2020 in San Francisco  (Photo by Justin Sullivan/Getty Images)

Gabriel Rotello.

As we watch the disastrous results of reopening and the surge in new infections, I keep thinking that we’re running a giant epidemiological experiment designed to illustrate how to make an epidemic as catastrophic as possible.

In an epidemiological nutshell, the resurgence is happening because we are largely ignoring the three fundamental factors that determine whether an epidemic grows or shrinks, namely Prevalence, Infectivity and Contact Rate. If you really want to bring an infectious epidemic like Covid-19 under control, you better try to address all three.


Prevalence is the percentage of the population that is infectious at any given time.

It’s important because, other things being equal, you will have a lot more transmission in a population where 1 in 10 people are infectious than in a population where only 1 in 10,000 are. The reason why epidemics always begin slowly is because at the beginning of all epidemics, prevalence is inevitably low. As more people get infected, increasing prevalence becomes like a snowball rolling downhill.


Infectivity is the statistical likelihood that a particular pathogen will actually be transmitted when an infectious person and a susceptible person come together.

Different diseases have different levels of infectivity. For example, the infectivity of measles and smallpox is incredibly high while the infectivity of most sexually transmitted diseases (like HIV) is so low you have to exchange significant bodily fluids to achieve transmission. Covid-19 is relatively high, though not as high as measles. Infectivity is obviously important because the more infective a pathogen, the easier and faster it will spread.

Contact Rate

Contact Rate is the rate at which infectious people come into contact with susceptible people in a given population.

It’s essentially the river upon which human-to-human disease transmission flows. The reason for its importance is pretty self-evident.

In a crowded city where people might come into contact with thousands in a single day, diseases have much more opportunity to spread than in a rural area where people might only come into contact with a handful of others.

Because Prevalence, Infectivity and Contact Rate are so important, they form the basis for the three main strategies we use to try to combat epidemics.

The main way to address prevalence (short of a cure or a permanent lockdown) is through testing, contact tracing and quarantine.

The purpose of these prevalence-based strategies is to find infectious people and temporarily remove them from the population, thereby reducing prevalence within that population.

The main ways to address infectivity (at least for respiratory diseases like Covid-19) are by wearing masks, washing hands, staying six feet apart, meeting outdoors rather than indoors, and so on. The idea behind these infectivity-based strategies is to reduce the chance of transmission when infectious people and susceptible people do come together.

This, by the way, is also the idea behind condoms to prevent HIV transmission, and also the more recent strategies for HIV prevention like PrEP, PEP and Treatment as Prevention.

All of these are ways to reduce infectivity per contact.

And finally, we reduce the contact rate itself by keeping potentially infectious and susceptible people apart. That’s why we had the shut-down. Contact rate is so critical that in the case of Covid-19, governments all over the world decided it was worth trashing their economies to bring the contact rate down.

It’s a blunt and painful instrument, but it’s vital if things are spiraling out of control.

So what does all this have to do with the big reopening disaster that’s happening now?

Think of it this way. By re-opening, we are not directly addressing or changing prevalence or infectivity. What we’re doing is increasing the contact rate.

Now you might think that this would automatically increase transmission, but not necessarily, at least if you do it right. That’s because prevalence, infectivity and contact rate work together synergistically, kind of like a seesaw. If one of these factors increases but the other two decrease, things might balance out and you might have a chance of keeping transmission from spiraling out of control.

For example, imagine that you increase the contact rate by reopening the economy. But at the same time, you reduce prevalence by aggressively testing and isolating infective people. And you also reduce infectivity by making sure everyone wears masks, observes the six-feet rule, gathers outdoors rather than indoors and so on.

In that case, the decreases in both prevalence and infectivity might balance the increase in the contact rate and you might avoid a resurgence.

True, it’s hard to balance this seesaw, in part because contact rate is such an important factor. But it’s possible.

Protestors along 3rd Street allegedly set a police cruiser ablaze as thousands take to the streets near The Grove in Los Angeles. (Photo by Troy Masters)

And, in fact, it looks like that may be why the recent protests following the murder of George Floyd didn’t turn into engines of infection in most places. The protests amounted to a sudden, drastic – but very temporary – increase in the contact rate for those who participated.

But in many places the protests followed months in which strict isolation had driven prevalence down to very low levels.

They also occurred outdoors rather than indoors, and most participants wore masks, both of which would powerfully reduce infectivity. Under those circumstances, the reductions in both prevalence and infectivity may have balanced the sudden, very temporary surge in the contact rate, and we avoided major transmission events.

But unfortunately, that’s not what’s happening with the reopening in general.

Under our current leadership vacuum, we are deliberately engaging in a long-term increase in the contact rate while making virtually no attempt to tamp down prevalence or decrease infectivity to balance things out.

In the absence of a cure or an endless lockdown, prevalence is reduced when you test, contact trace and isolate infectious people. But while testing has increased, there are no nationwide or even statewide programs to isolate infectious people, which is the main benefit of testing. In some countries that have kept transmissions low, people who test positive are required to isolate at home.

Not urged to, required to.

They are constantly called and visited and otherwise monitored by public health workers, provided with food, medicine and other services, and repeatedly re-tested until they clear the virus.

Countries like China go even further.

People who test positive are sent to so-called ‘fever clinics’ and are required to stay there until they test negative, usually about two weeks. Some countries do a combination. If you live alone, they require you to isolate at home. If you live with others whom you might infect, they send you to isolation clinics.

Testing alone doesn’t do much unless you provide a safe, comfortable, humane and cost-free way to briefly isolate the infectious. But we’re not doing that.

As a result, we’re not addressing prevalence at all.

And when it comes to infectivity, reckless politicians are actually encouraging people to abandon masks, ignore social distancing, gather indoors, etc.

Under these circumstances, what’s happening with the reopening is this. We are increasing the contact rate significantly by reopening, but we’re doing nothing to tamp down prevalence and we’re actually increasing infectivity.

This is virtually a textbook definition of how you stoke an epidemic.

All this being true, I’m not particularly optimistic about the future even in places like New York, New Jersey and Connecticut that seem to be doing well at the moment. The tri-state region is almost certainly doing well because the lockdown and social distancing were extremely strict, which reduced prevalence to very low levels. But as the region reopens without a way to quickly identify pockets of infection and isolate the infectious, and without mandatory adherence to masks and other methods of reducing infectivity, transmission will eventually go back up. It’s just going to take a bit longer.

The good news is that epidemiologists know what we need to do to bring transmission under control. These principles have been well understood for over 100 years. And in places like Singapore, Hong Kong, South Korea, Vietnam, New Zealand, even China itself, where the virus has been largely eradicated, success has come by addressing all three factors of Prevalence, Infectivity and Contact Rate.

The results have been impressive. Life has returned to a semblance of normal and the inevitable minor outbreaks are quickly identified and stamped out.

The question is, do Americans have the ability – or the will – to do that here?

And do leaders even understand what we need to do?

For example, I keep hearing well-meaning politicians talking about the importance of testing, which is fine. But I almost never hear them go on to stress the importance of isolation, which is the main point of testing.

In the end, without a clear understanding of how epidemics work, we exist at their mercy.

And as humanity has learned repeatedly since time began, epidemics have no mercy at all.

—  Gabriel Rotello is author of the 1997 book, “Sexual Ecology: AIDS and the destiny of gay men,” a book about the epidemiology of HIV and co-founder of OutWeek Magazine. He is currently a television writer, producer and director living in Los Angeles.


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