Are the vaccines interchangeable?

I have a question that some of you may be able to help with.

My DS18 got his first dose of Pfizer on May 23. Currently, I was able to snag (yes, around here, you have to use all your FPP skills to get appointments) a second Pfizer dose appointment for July 18. Today, I found out that there’s another clinic that’s just opened up offering Moderna appointments as early as next week.

There’s been a shortfall of Pfizer delivered in Canada this week and may cause either cancellations or switching of vaccine brands anyway. Our doctors are saying the two are interchangeable, but I don’t know if that’s just a political line because they can’t get what we all got for our first doses or if it’s actually true. I don’t think the CDC approves of mixing and matching, so that makes me leary.

My question, should DS keep his July 18 appointment and hope Moderna isn’t substituted anyway (or appt is cancelled) or should I be proactive and book him for a Moderna next week.

What would you all do?

Thanks for your help.

They are not interchangeable. If you switch to Moderna, you will need a second Moderna.

Exception would be J&J. Since that one is a single dose, you can switch to that and not need the second. But J&J isn’t as effective, either.

Here is the CDC page on the topic.

Interim Clinical Considerations for Use of COVID-19 Vaccines | CDC

“The safety and efficacy of a mixed-product series have not been evaluated. Both doses of the series should be completed with the same product.”

I know the CDC can be picky about US testing. Has mixing and matching been tested anywhere?

Unless it has, I would wait.

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Here is the detailed guidance from Canada and the studies they are citing to support mixing.

Archived 12: Summary of NACI rapid response of June 1, 2021 -
“Recent studies on the safety of and one study on the immune responses produced using mixed COVID-19 vaccine schedules provide the evidence for vaccine interchangeability - a study from Germany and a clinical trial from the United Kingdom report on the safety of mixed schedules, and a Spanish trial reports both the safety and immune responses produced from mixed COVID-19 vaccine schedules.”

Archived 12: NACI rapid response: Interchangeability of authorized COVID-19 vaccines [2021-06-01] -

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Actually, that’s not the CDC clinical guidance:
Using the above strategies, every effort should be made to determine which vaccine product was received as the first dose to ensure completion of the vaccine series with the same product. In exceptional situations in which the mRNA vaccine product given for the first dose cannot be determined or is no longer available, any available mRNA COVID-19 vaccine may be administered at a minimum interval of 28 days between doses to complete the mRNA COVID-19 vaccination series. In situations where the same mRNA vaccine product is temporarily unavailable, it is preferable to delay the second dose (up to 6 weeks) to receive the same product than to receive a mixed series using a different product. If two doses of different mRNA COVID-19 vaccine products are administered in these situations (or inadvertently), no additional doses of either product are recommended at this time. Such persons are considered fully vaccinated against COVID-19 ≥2 weeks after receipt of the second dose of an mRNA vaccine.

Interim Clinical Considerations for Use of COVID-19 Vaccines | CDC


This article that just came out today basically answers your question.

To summarize:

  1. Getting a combination of vaccines may actually help better protect against variants.

  2. Combinations are being studied in several countries including Spain, Germany, and the UK.

  3. Both the CDC and the Canadian health agency say you can go with a different vaccine for the second dose in exceptional circumstances like a shortage of the type of vaccine you received for the first dose.

  4. They don’t think that it causes any particular adverse reactions.

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Add to this, those of us in Canada who were given the AstraZeneca vaccine for first dose can either get a second AZ or one of the mRNA ones. I will have one AZ and one Pfizer (if they don’t run out and have to give me Moderna) at 12 weeks.

So, while I’ve heard this is actually good, but I do wonder just how ‘safe’ I am compared to someone who got both of either the AZ or an mRNA.


Well…I am one of those guinea pigs in Western Canada. I was give AZ for the first does. It took me down pretty good for a few days. 3 Days ago (8 weeks and 4 days after the first dose) I got Pfizer for the second. I did not have the same extreme side effects and felt pretty good after a day of being lethargic.

I spoke to my DBiL who is a nurse and he looked into it. He was citing some of the same studies you have mentioned above. (That is may be a good thing).

Now I am just waiting for my superpowers to emerge so I can save us from the TVA! :rofl:


Interesting. That is a 180 degrees from what they were telling us earlier. Hard to keep up!

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I wonder if @Beth33 has any thoughts???

Makes sense that introducing multiple mechanisms of action with the two doses yields a more robust antibody production, per recent studies, like the Spanish one mentioned with ~660 people where AZ/Pfizer had double the antibodies of AZx2. mRNA and viral vectors will stimulate the immune system slightly differently and can add up to a more efficient response. There just wasn’t the safety info before that seems to be there now, since earlier studies had to focus on the same vaccine for both doses.

Not worried about the vast majority of us in the US who have had the same for both doses. Protection is easily still good enough.

Thee has been testing on using different vaccines. And some people have received different ones in both the U.K. and Canada, where they restricted the use of AZ in younger people. Or even where people got severe (requiring hospitalisation) side effects from one on their first shot.

I would say if you had AZ for the first you may well end up more protected with a different second shot, especially when the Delta variant spreads. AZ even after 2 shots is still only around 66% effective against Delta.

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I’m trying to find a form of words that is neither offensive nor inflammatory in response to this.

First, it’s not true.

Second, even if it is true, it’s misleading.

There is no single effectiveness metric that is universally applicable. It’s not simply a matter of “you got this jab, therefore you are this protected”. Other factors play a role. A significant role.

I grow weary of the constant drip, drip, drip of anti-AZ propaganda. But, sure, I have skin in this game. I’m doubly AZ-vaccinated.

Then maybe don’t post clinical advice on a public forum.


Oh, I forgot to mention:

This statement isn’t even very meaningful. If you are infected with COVID there isn’t only one possible outcome. There are many. You may be asymptomatic, you may feel crappy for a few days, you may be admitted to hospital, or you may die. The likelihoods of these things for those who are vaccinated are not all the same. As well as not being the same for all people.

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I also had AZ. These figures were published in the UK press on the Delta variant. Which is driving a third wave in the UK right now.

Against hospitalisation and death it’s still very high. But of the 76 deaths of delta so far 26 of them were in fully vaccinated people. A stark figure. People are still getting it despite being vaccinated.

I am not for a minute saying it isn’t effective. Yet fully vaccinated people with AZ and Pfizer are getting COVID and dying. But in very very small numbers.

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Wow! We were lucky here in NYC - only had to use super skills to get the first appointment. They had us make the second while we were waiting in the holding room during the 15 minutes after the shot. That way you knew you would get an appointment for the right one (it auto-filtered to whichever one you had gotten) within the time frame.

It amazes me that with the time-sensitive nature of getting the second dose everywhere didn’t figure that process out!

Good luck to all who have to do the gauntlet twice!


Yeah, that’s exactly how we did it here as well.


When we got our shot, there was a questionnaire. One of the questions was if you had received a dose of another shot (meaning Moderna in this case). If you answered yes, then after the shot, they would still have you schedule a second dose of the Pfizer. That was the clinical recommendation only a few weeks ago, which I was repeating.

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I have not heard that they are not interchangeable. If the first dose if Pfizer, I understood that you must get the second dose of Pfizer. The studies were done with the same doses. I’m not aware of any studies that are mixing brands here in the US. When my family got shots, including kids 17 and 12, the second dose was scheduled at the first dose appointment.

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