2021-06-04 BC

AZ+AZ vs. AZ+mRNA

Ed: updated on 7 June to include the small Saarland study.

Ed: updated on 6 July to include the Cologne study.

A fair number of Team AZ members are wondering if they should choose an mRNA vax or AZ for their second dose. Below is what I know; this article also gives advice. Mine is more complete and more BC-focused, theirs is probably easier to read/digest.

There have been six studies on AZ+Pfizer: a big one in the UK (called Com-CoV), a Spanish one, a small German study in Berlin, a small German study in Saarland, and a tiny German study in Ulm, and a medium German study in Cologne.
Side effects:
- The Com-CoV found that AZ+Pfizer gave stronger effects than AZ+AZ. (The side effects were still mild, meaning “no lasting damage and over pretty quickly”, but possibly quite unpleasant.)
- The tiny Ulm study said that the side effects were “in line with previous reports” and that the Pfizer second shot side effects weren’t as bad as the AZ first shot. (I believe that with AZ+AZ, the first dose’s side effects are worse than the second.) They did not compare AZ+AZ directly to AZ+Pfizer.
- The small Berlin study said basically the same thing as the tiny German study.
Effectiveness:
- The Com-CoV study alas is not going to publish their effectiveness results for about a month, unfortunately.
- The Spanish study found that AZ+Pfizer gave 7x the number of antibodies that AZ+AZ did.
- The tiny Ulm study said that AZ+Pfizer neutralized the virus better in test tubes than AZ+AZ did. It’s difficult for me to quantify how much because I am not well versed in antibody titer science.) I think that it says that AZ+AZ does better against the Beta and Delta strains. (I am not a biologist, so am not sure.)
- The small Berlin study found that the antibody response and T cell reactivity was significantly higher for AZ+AZ vs. AZ+Pfizer.
- The small Saarland study found that AZ+Pfizer and Pfizer+Pfizer made ten times more antibodies than AZ+AZ, and that AZ+Pfizer was “slightly” better than Pfizer+Pfizer. (Sorry, I don’t know what “slightly” means.)
- The small Saarland study found that AZ+Pfizer gave a stronger T-cell response than AZ+Pfizer or AZ+AZ.
- The medium Cologne study found that AZ+Pfizer gave “a striking increase” antibodies compared to AZ+AZ and even Pfizer+Pfizer.
These studies all look at what happened in test tubes. They do not compare infection rates in humans. Also, it is important to understand that people measure antibody levels because it is easy. However, my understanding (which might be wrong!) is that the antibodies go away relatively quickly, and that what is really important is how well the T cells remember the virus and make antibodies next time. That’s a lot harder to measure, however, so it is much rarer.
- It’s really hard to do head-to-head comparisons because different clinical trials are run at different times in different places, but Moderna, Pfizer, and AZ are all comparable, all “good enough” against almost all variants. Unfortunately, AZ is absolute shit against Beta (the South African variant). It is stunningly bad. It’s so bad that in the middle of a wave, South Africa packed up all of its AZ and sent it to places which didn’t have a lot of Beta. We have basically no Beta circulating in BC, so that isn’t a big issue here, but if you ever want to travel, it might become an issue for you.
- AZ uses an “viral vector” — an inactivated adenovirus — to sneak its payload into your cell nucleus. With some other viral vector vaccines, they have found that the body can get immunity to the carrier virus and attack it, meaning that less of the payload gets into your nucleus. (This is why the Sputnik V vaccine uses different adenoviruses for the first and second doses.) I don’t know if this happens with AZ ever, I’m not sure if anybody knows, but it’s possible.
Availability:
- In BC, we only have about 125K doses of AZ on hand, and about 272K on Team AZ who need a second dose. We are supposed to get another 135K from South Korea by the end of June, which still leaves us 10K short if everybody takes AZ. NB: We got 34K doses on 16 June 2021. We’ve got plenty now.
- They say they are offering second doses in the order that we got our first doses, so if all of Team AZ takes AZ for their second, they’ll run out at about the time they hit people who got their first shot on 17 April.
- At their peak, the BC pharmacies delivered 13K doses/day. I expect demand to be very high, so let’s assume we’ll do 12K/day. At that rate, we’ll run out of our current inventory around June 15th (which would roughly correspond to the first-dose people of around 17 April if everyone takes AZ). You would then need to wait until the South Korean shipment comes in.
- BC has tons of Pfizer and will have tons for the foreseeable future. Pfizer has been extremely reliable.
Time: You probably will have to wait longer to get an mRNA dose. This will depend on how many people take first doses and how many of Team AZ take mRNA, but I am spitballing that you will wait three or four weeks longer to get mRNA. (And I predict that everyone will get their second dose before September.)
Safety:
- AZ has a risk of blood clots, but the risk of a clot after the second dose is about 1/10 of that from a first dose. In the UK, there have been zero cases so far in people aged 18-49 and 1.4 cases per million for those over 50.
- Israel has seen some case of heart inflammation in young (mostly) men after getting a Pfizer dose. It is rare and they have not proven conclusively that it is connected to Pfizer. (It might just be that people are socializing a lot now, so getting random viral infections, which can lead to heart inflammation.) The cases have been 90% men, mild and resolved quickly.
Experience/Data:
- There have been very few studies of AZ+Pfizer compared to AZ+AZ or AZ+Pfizer studies. I don’t know of any AZ+Moderna studies. There have been probably only hundreds of people (maaaybe thousands) who have gotten AZ+Pfizer, compared to millions who have gotten AZ+AZ.
- There is no study which calculates the vaccine efficacy of AZ+Pfizer, only studies in test tubes.
- To be fair, I have never heard of anybody having any concern about mixing brands for any other kind of vaccines. I’ve gotten at least four different measles shots in my life, and nobody has ever asked me which brand they were.
Robustness: When training, diversity is a Good Thing. If you only ever hear one person when you are learning Italian, it will be harder for you to understand a random Italian when you encounter them. With the vaccines, you are training your immune system to fight of slightly different attackers. This ought to prepare you better for the attack of some random COVID variant.
Affection: You might get a little boost of pride from having some brand. Maybe you are proud that women (like Katalin Karikó, Özlem Türeci and Kizzmekia Corbett) have been instrumental in their development. Maybe you’re proud of the Hungarian (or Turkish or Black or UK or German or Oxford) contributions. Maybe your grandmother was named Astra. It would be easy to scoff, to say that’s a silly reason to choose one vaccine over another, but if it makes you walk a little taller, that is actually worth something. I wouldn’t make it the biggest factor in your choice, but if you need a tie-breaker, this is as good as any.

Me, I am going for the mRNA.

I am very low-risk, so I can wait.
I believe very, very strongly in the value of diversity, and the studies on efficacy, while they do not prove that mixing is better than matching, they certainly hint in that direction.
I am concerned about AZ’s crappy response to Beta.
I have some affection for the mRNA vaccines.

Ed: I got Pfizer on 18 June 2021. I was tired for a day, then had a headache for a day (which might not have been related to the vax). Go Team PfAZer!