Wait, is omicron BA.5 really as contagious as measles? Not so fast

If you’ve been surfing some corners of COVID Twitter over the last week or so, you might have seen stories and accompanying viral threads about how the BA.5 subvariant of omicron could be “one of the most infectious viruses known to man,” perhaps even as contagious as measles.

However, some experts say such comparisons are problematic, in part because they’ve been relying on a flawed calculation for the subvariant’s transmissibility.

In a story posted to the Conversation earlier this month, Adrian Esterman, a professor of biostatistics at the University of South Australia, estimated that the BA.4 and BA.5 subvariants had an R0 of 18.6, “similar to measles.” Esterman based his calculation in part on research showing that those variants had a significant “growth advantage” over the BA.2 variant, which itself had a growth advantage over the BA.1 variant.

Esterman’s story was republished in publications such as the Guardian and repeated elsewhere in mainstream media. But some fellow public health experts took issue with the measles comparison, saying on social media that the R0 number cited in the story was “just outright wrong.”

So what’s the issue? Scientists use R0, or “R naught,” to estimate disease infectiousness in a population with no protection, such as immunity from vaccines or prior infections, but at this stage in the COVID-19 pandemic, most people have one or both of those things.

“R0 is a tricky concept,” Natalie E. Dean, an assistant professor of biostatistics at Emory University’s Rollins School of Public Health, tells Fast Company. “It is particularly important early on in an outbreak for understanding how much a pathogen might spread. But as time progresses, it becomes harder to interpret.”

In a recent article in the Atlantic, virologist Trevor Bedford of the Fred Hutchinson Cancer Research Center was quoted as saying it’s “fully wrong” to describe BA.5 as on par with measles. Although the variant is clearly very contagious, it doesn’t need to be the most contagious virus in history to spread as far and wide as it’s spreading right now—nor is contagiousness the only factor in how widely a virus can spread. One reason it’s spreading is because of its ability to evade immunity protections from vaccination or prior infection. This doesn’t mean that we have zero protection from BA.5, but the subvariant definitely appears to be better at getting through those defenses.

“The reality is that BA.5 does not need to have an R0 of 18 to be infecting a lot of people,” Dean says. “Getting people up to date on their vaccines/boosters is very important as numbers rise. Over time since vaccination . . . our protection against infection wanes. But that protection against severe disease holds up over longer time scales, and that provides some reassurance.”

Reached for comment by Fast Company, Esterman stood by his measles comparison, noting that none of his critics “were willing or able to provide estimate of R0 for BA.5.”

“Since BA.5 is far more transmissible than BA.1, an estimate of 18 is not totally unreasonable, Esterman said. “The main thrust of my argument was that BA.5 is probably now as transmissible as measles. That argument holds true even if BA.5 has an R0 of 12.”

He added, “At least my article has promoted robust discussion on the use and calculation of R0, which can only be a good thing.”

According to the most recent estimate from the Centers for Disease Control and Prevention (CDC), BA.5 now makes up about 65% of all cases in the United States. Exaggerated headlines notwithstanding, it’s a serious public health risk however you look at it.

“[The] comparisons with measles are not particularly helpful,” Dean says, pointing out that “measles has different properties—long-lasting protection after infection/vaccination.”

“That being said, both can be true,” she adds. “Something can be more immune evasive and more transmissible. But it is important to recognize that some of that advantage comes from the evasion.”