Should You be Afraid of Delta?

July 27, 2021 By Alan Salzberg

The latest variant, called Delta, is responsible for most US infections now.  What can we expect?

Here's what the data tells us.

1) Delta makes up more than 80% of new infections as of a week ago (see  Delta is much more contagious than the "original" COVID or other variants.

2)  Chances of Infection are rising.  There are many break-through infections with the Delta variant, because the vaccines are less effective.  How much less effective is unclear.  Recent Israeli data shows effectiveness against infection of only 40% for the best vaccines ( see Israel study here) but British data showed nearly 90% effectiveness: see British study here ).  I cannot explain the difference, but it could be that Israel's vaccines happened earlier and their effectiveness is waning.  However, even in Britain, the rate of infection reached nearly 50,000 cases per day last week (only 20% off its peak of 60,000), and that means many many vaccinated people are getting the variant.  Based on US confirmed daily caseload increasing to 100,000 soon, and assuming a 5-1 ratio of infected to confirmed, this might mean your daily chances of getting COVID are on the order of 1 in 1,000 if un-vaccinated and 1 in 3,000 if vaccinated. Because of the concentrations of vaccinated/unvaccinated people (they tend not to hang out together), these odds are likely lower for vaxed and higher for unvaxed.

3) If you're vaccinated, you are not likely to get very sick.  Studies have consistently shown around 90% or more effectiveness against serious disease or hospitalization (see, for example: this British study, which is confirmed by an Israeli study). In other words, vaccine effectiveness against hospitalization and death only dropped a little for the delta variant (it was closer to 95% for other variants).  Thus far (pre-delta), around 800 vaccinated people have died of COVID.  Given the 160 million who are vaccinated, that amounts to about 1 in 200,000.  However, these people have only been exposed (vaccinated) an average of around 3 months.  The average number of cases during April, May and June was around 35,000 cases a day.  If we go they way of England and our cases increase substantially, we will have averages of triple that, or more.  Given slightly lower protection (lets assume from 95% to 90%, which is consistent with the data), that could mean 4,800 deaths among vaccinated people by October.

That's about 1 in 30,000 across all age groups.  Age continues to be a huge factor in deaths and hospitalizations.  Therefore, young people continue to have near 0 risk of serious COVID, and young with vaccination means 40-50 year-olds and younger, whereas it used to mean 20-30 year-olds and younger (this is because vaccination reduces chances of serious COVID close to ten-fold, which is roughly equivalent to being about 20 years younger).

4) If you're unvaccinated, Delta is just as bad (but probably not worse) in terms of its seriousness once you get sick.  This means that, as before, if you are young (<50), you're probably going to be fine--your chances of getting sick are substantial but your chances of getting hospitalized or dying are very small (in the worst weeks of the pandemic, the chances of hospitalization for 18-49 year-olds was never more than 1 in 10,000).  If you are older and unvaxed, your risks may return to levels they were at the height of the pandemic (weekly hospitalization rates of 1 in 1,000 for those >65).

5) At the population level, cases in the US may near prior peaks, but hospitalizations and deaths won't be even close, due to vaccinations.  This last conclusion is purely from British and Israeli data (see ), which have seen large increases in cases but only relatively small increases in hospitalizations and deaths.

6) At some point, delta will die out and we will see herd immunity through Delta infection and immunization (even at 50% effectiveness, immunization makes a big dent).  I predicted we'd get to herd immunity pre-delta with 50% vaccination rate (see here ) and that prediction proved true.  With Delta, we'd probably need a combination of infection and vaccination that is substantially higher.  It's difficult to know, but it's clear we are not there with 50-60% vaccination or we wouldn't be seeing the increases in cases that we are currently seeing.

It seems the current summer wave may only last a few more weeks, if Israel and Britain are good indications.  But we could well get a winter wave that is equally bad or worse. Those who are vaccinated will not be bothered much but those who are unvaccinated incur substantial risk.

7) Note that regarding vaccinations, I am talking about Pfizer/Moderna, which are the lion's share of US vaccinations.  For J&J the numbers are almost surely worse, but almost surely better than no vaccine.  Thus far, there have been only a few small studies of J&J vs delta.

So overall, Delta is not at all scary if you have been vaccinated.  If you haven't been vaxed, you have real reason for concern.