The skinny on breakthrough cases in the Delta era

My wife (and therefore possibly me too) has had to get tested for COVID because a double-vaccinated friend of hers, whom she saw briefly last week, has tested positive. (UPDATE: Negative. Panic Over.) Welcome to the Delta era.

It's no secret that double-vaccinated people are catching the Delta variant of the virus. Indeed, these so-called "breakthrough cases" are expected to increase in frequency as more and more people are vaccinated (a still small percentage of an ever increasing number). Positive tests in vaccinated people still only account for about 1% of cases overall, although in recent weeks, when the more infectious Delta variant has been dominant, this is creeping up to 10%.

The vaccines are still doing their job, though (bear in mind that there are now three times as many vaccinated as unvaccinated people in Canada, and yet their infection rate is ten times less, not three times more). And, in particular, the vaccines ensure that those who do catch it have a much milder experience, and very few end up in hospital (and even fewer dead). Those vaccinated people who do end up hospitalized also tend to have other risk factors (e.g. old age, compromised immune systems, etc).

It may also be the case that some of the older (and more vulnerable) people who received their vaccines early may becoming due for a booster shot, as we now know that the efficacy of the vaccines starts to wane after several months, although this should not yet be the case for the general population. Protective T-cell responses to the virus seems to maintain their efficacy, though, even as antibody counts fall.

There has been a lot of media attention to the fact that fully-vaccinated individuals can carry as large a viral load as the unvaccinated, suggesting that they are just as contagious as unvaccinated people, but there is nuance here too. For example, the viral load falls off much quicker in vaccinated people, so they may be contagious for just a day, rather than five days. The vaccinated will also tend to have less of the live virus in their viral load, and so will be less contagious for that reason too.

All of which points to what we already knew: we need more people vaccinated, and we need to continue with other protections too, like mask-wearing and limiting large gatherings and indoor groups.