Over the past two years, we have witnessed evolution in action, with new variants of the coronavirus becoming increasingly contagious. But where will it end? Is there a ceiling that viruses reach where they can鈥檛 become any more contagious?
Viruses tend to become more contagious over time because natural selection favours mutations that increase their ability to spread, says at the Peter Doherty Institute for Infection and Immunity in Melbourne, Australia, who is a member of the World Health Organization鈥檚 Technical Advisory Group on SARS-CoV-2 Virus Evolution.
Measles is perhaps the most contagious virus we know of. Its basic reproduction number 鈥� or R0 鈥� is 12 to 18, meaning that in a na茂ve population with no immunity from prior infection or immunisation, like a group of young children who haven鈥檛 yet received the measles vaccine, one individual could transmit the virus to 12 to 18 others.
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Measles has been around for 2500 years, so why hasn鈥檛 it evolved to become even more contagious and push its R0 value up further? 鈥淭here are constraints within the measles virus itself on how much it can evolve, so it doesn鈥檛 evolve anywhere near the rate of other viruses like coronaviruses, influenza and HIV,鈥� says at the University of Western Australia in Perth.
That is why we typically need to be vaccinated against measles only once in childhood to protect us for life, because the virus doesn鈥檛 evolve new variants that can evade vaccines, she says.
By contrast, SARS-CoV-2, the coronavirus that causes covid-19, can evolve with ease. The original strain that emerged in Wuhan, China, in late 2019 had an R0 of about 2.5, while the delta variant pushed the R0 up to about 6.
We don鈥檛 know the R0 of the omicron variant because, by definition, it must be measured in a na茂ve population and it is now difficult to find groups without any immunity from previous SARS-CoV-2 infections or vaccination. However, omicron does seem to be more contagious than previous variants, possibly because it has a different way of infecting cells.
鈥淲ith the original Wuhan strain, it was just starting to nestle into the human immune system and adapt to it,鈥� says Druce. 鈥淣ow, with omicron, it鈥檚 evolved and is far more efficient at transmission.鈥�
New SARS-CoV-2 variants that pop up in the future could be even more contagious than omicron, and perhaps even more so than measles, says Imrie. 鈥淭here鈥檚 no reason to think not.鈥�
One reason why measles has the highest known R0 may simply be because it is so well studied, owing to its severity, says Imrie. Common cold viruses, for example, may evolve super-contagious variants that sweep the globe, but we wouldn鈥檛 know because their typically mild symptoms mean we don鈥檛 usually need to monitor them closely, she says.
Then again, Druce says he would be surprised if viruses could get much more contagious than measles because it combines so many elements that make it a perfect spreader. For example, people infected with measles shed large amounts of the virus and can spread it for four days before realising they are sick. It transmits via coughing and sneezing and can hang around in the air for up to 2 hours. And it is a uniquely human virus, meaning it is perfectly adapted to infect us.
Viruses that are highly contagious do hit an upper limit of transmissibility because of the way we develop population immunity, says Imrie. 鈥淕enerally, viruses reach a point where they become less contagious because enough people have developed immunity from prior infection or vaccination, so they can鈥檛 spread as easily,鈥� she says.
If a new variant emerges later that can evade this immunity, the same pattern will repeat, says Imrie. This is what has happened with omicron: it initially spread explosively by evading immunity from vaccination and infections with other variants, but now cases are falling in some places as more people develop immunity specifically to omicron.
Now we just have to hope that when the next variant of SARS-CoV-2 emerges, it doesn鈥檛 pack a double punch by causing more severe disease as well as being more contagious, says Imrie. 鈥淭hat鈥檚 the big worry.鈥�
