How Contagious Is Delta, Really?

Delta is more contagious than the original COVID-19 virus, and it’s even more contagious than the other variants of concern. That’s why the CDC in the US has recently recommended masking up in some situations even if you’re vaccinated — as many public health experts have been urging ever since Delta started spreading.

So just how contagious is it? A recently leaked CDC document contains some new data that puts it into perspective.

The CDC slides show a rough estimate of Delta’s contagiousness and fatality rate, drawn on top of this New York Times graph from earlier in the pandemic.

Delta might be slightly more deadly than original COVID (SARS-CoV-2 is the official name of the COVID virus), but importantly it is far more contagious. The common cold and various flu viruses are all roughly as contagious as original COVID, but Delta is substantially more so. Not as much as measles, a classic superspreader, but in about the same range as chickenpox.

If you’re old enough to remember chickenpox outbreaks as a kid, that sucks. I got chickenpox just a few years before the chickenpox vaccine came out, and as soon as one kid in our neighbourhood came down with it, our parents knew that chances are everybody would have it sooner or later.

Are vaccinated people spreading Delta?

If you’ve heard that vaccinated people shed just as much virus as unvaccinated people, that’s true, but only in the context of people who actually get sick. People who are vaccinated are very unlikely to get Delta, or any COVID variant. In the rare cases that they do, that’s when they have the same viral loads as people who are unvaccinated and caught the virus.

People who are vaccinated and who have not contracted an infection with Delta aren’t spreading the virus and aren’t shedding viral particles, or spike proteins, or anything else. The problem isn’t the vaccine, it’s just that the vaccine isn’t perfect.

“Delta variant breakthrough cases may be as transmissible as unvaccinated cases,” is the way the CDC presentation puts it. Before Delta, breakthrough cases were thought to be possibly less transmissible than cases in unvaccinated people who came down with COVID.

To be totally clear about what the vaccine is doing for us, the CDC’s data shows that the vaccines work well. Currently, unvaccinated people have eight times higher incidence of COVID than vaccinated people, and they have a 25-times-higher risk of death from COVID.

The CDC presentation says that “[g]iven higher transmissibility and current vaccine coverage, universal masking is essential to reduce transmission of the Delta variant.” The CDC chose a less drastic message in their announcement the other day, recommending masks depending on your personal situation and the transmission level in your county. But it’s also important to note that if more people are vaccinated, Delta’s spread will be slowed and masks may not be necessary when cases are low enough.

For more information please refer to the Department of Health website.