Explainer: how contagious can a disease get?

After Delta continued to spread despite harsh lockdown restrictions, it seemed absurd that COVID-19 could get more contagious. But then the Omicron variant appeared – estimated to be three or four times more infectious than Delta.

Does contagion have an upper limit? How infectious can a pathogen get?

Sanjaya Senanayake, an associate professor of medicine at the Australian National University, says that we’re not really sure. Diseases are often compared to measles, widely considered to be one of the most infectious things we know about.

“Amongst our most important communicable diseases, or infectious disease of public health importance, measles has pretty much stood there alone,” says Senanayake.

The infectiousness of diseases is measured by the basic reproduction number (R₀). The R₀ is the number of people one infected person would transmit a disease to, on average, assuming there were no specific protections in place (like vaccination or social distancing).

Estimates for the R₀ of measles vary, but Senanayake says it’s between 15 and 30.

“One person with measles [could] infect anywhere from 15 to 30 susceptible people – that’s people who aren’t vaccinated, et cetera. But we haven’t seen anything beyond that. So it is hard to know how far it can go.”

It’s hard to tell exactly what the R₀ of Omicron is – because it’s so new, because there are lots of undetected cases, and because it’s hitting populations that are already vaccinated and undertaking social distancing measures. Some estimates say it could be below 10, while others are much higher – if it’s three or four times more infectious than Delta, which has an R₀ of between three and eight, that puts it in the realm of measles.

There’s no strict biological limit on how high R₀ could be.

“Theoretically, it could be eight billion,” says Senanayake.

“If there’s one infected person who ran around the world very quickly, while infectious, it could infect everyone. But obviously, in practical terms, that won’t happen because a single infected person will have limited contact with other people.

“Even if they get on a plane and go to another country, they’re not going to meet millions of people while they’re infectious. And, of course, you have to remember while they’re infectious, they’re probably likely to be not feeling too great and not socialise as much as they normally would.”

And while there’s not much we could do about the R₀ or the inherent infectiousness of a pathogen, at this point we all know there are things we can do about the actual levels of contagion.

The effective reproduction number (R_eff) represents the average number of people an infected person has actually passed a disease on to in a given population. The R_eff of COVID-19 in Australia has typically been much lower than the R₀, because of public health restrictions and vaccination.

“If you look our Omicron reproduction numbers throughout this outbreak, it definitely hasn’t been sitting around 12 or 15, or 20, or anything like that in Australia. And that’s because it’s the effective reproduction number we’re measuring. It’s a population that’s highly vaccinated, and a population in which certain restrictions have been put in place,” says Senanayake.

“Without the vaccinations and without some COVID safe measures, [Omicron] would have been a lot worse in terms of cases and hospitalisations and deaths.”

This is a salient reminder to get vaccinated, and get your booster shot, if you haven’t already. Two shots of a vaccine provide protection against getting seriously ill if you’re infected with Omicron, while a booster also provides some extra protection against getting infected in the first place.