In my country it has been decided to postpone the second dose of the Sputnik V vaccine, "to vaccinate more people". They've decided this based on no study but solely in the belief of the vaccine creators who stated "we believe the second dose of the vaccine can be postponed up to 3 months because other similar vaccines made it and it worked". Now it's been evident that they arent going to make it in time for the 3 months either. People could receive the second dose in 4 months or so.

What are the possible effects of getting the second dose of a coronavirus vaccine out of the expected time? A lesser immunity effect than the expected? No additional effect at all? Expiration of the immunity created by the first dose? The 2-3 doses scheme have been done in the past with other vaccines, can something be concluded by seeing how it works in them?

  • One thing to keep in mind is that the immune system doesn't have a specific deadline to be boosted by a second dose. In real life, you get re-exposed to diseases at various time intervals, and the immune system successfully boosts your immunity. Thus, the postponement takes into account what's known about how the immune system works (not just made up out of thin air).
    – Armand
    Jun 9 at 13:40

Canada decided to delay second doses of the three main vaccines in use here: Pfizer, Moderna, and AstraZeneca. The motivation was to get more first doses into people. The math there was

100 people get first doses, about 70% protection -- 70 protected people

50 people get both doses, about 95% protection -- 48 protected people.

Since 70 is more than 48, that was the public health choice made.

Now what you're asking is, when it comes time for second dose, will some people now get 90% instead of 95% because it was delayed?

The early studies on second doses did them really quickly. 4 weeks, even 3. But there were other studies with longer gaps and there was real world data with longer gaps where for various reasons people waited 12 weeks or 16 weeks. While many expected waiting would make it worse, that's not what happened. Waiting improved protection. People were more protected 12 weeks after their first shot (without a second) than 3 or 4 weeks after it. Then yes, it goes up even further when they get that second shot. (Quotes later in this answer.) They seem to show that 12 weeks is the "sweet spot" for a second dose, but the truth is from a public health point of view, it's not about maximizing one individual's protection to 96 or 97 rather than 95, but about the overall population protection.

Here in Canada we were originally scheduled for second doses 16 weeks after firsts, but it is now changing to 12. This is not so much because you get better protection at 12 than at 16 as it is that Delta does much better on singly-vaccinated people than the other variants do, so it's important to get a lot of people onto second shots before Delta takes hold here as it has in the UK.

This CBC news article quotes some experts:

"There is no question that for the whole of Canada, from the perspective of lives saved, that giving single doses to people and asking them to defer their second dose was the best idea," said Dr. Allison McGeer, a medical microbiologist and infectious diseases specialist at Toronto's Mount Sinai Hospital.

A new Canadian preprint study, which has not yet been peer reviewed, found the Pfizer-BioNTech and Moderna vaccines' effectiveness grew from 48 per cent 14 to 21 days after the first dose to 71 per cent after 35 to 41 days.

So, I have no numbers about Sputnik's effectiveness after 12 weeks vs after 16. I am just showing you that the decision isn't based on that. It's based on vaccine supply, and the effectiveness of a single shot against the variants that are currently circulating. If you think that you should get a second shot now rather than someone else getting their first, taking two shares from a limited supply, let me give you quote from Dr. McGeer from that article:

"Of course I want my two doses of vaccine. But if the price of that is somebody else maybe dying because of it — that's just not OK. I think we can all see that."

  • well... it's not me the one I care about, it's my father who has like 8 comorbidities (heart operation, diabetes, high pressure, ex smoker, etc.). Supposedly the 2 doses scheme in time would make a 100% effectiveness against death (not exactly true since we know cases of people who died but that was what the original study stated ) . We wonder how much he could make in times of peak of cases if he has the 2 doses and what he should avoid making for his safety. For a person like him having the maximum immunity or not makes a big difference.
    – Pablo
    Jun 9 at 15:20
  • Here, such people are already qualified for second doses. We have a parallel set of schedules, one based on age and one based on other health issues such as diabetes or being pregnant. Advocating for that where you are would be useful. The main issue is not whether the second dose gets him to 95 or 96%, it's that he needs his second dose as soon as the supply is there. Jun 9 at 15:30
  • here they (supposedly) have priority but in practice they vaccinated corrupt 20 years old law makers and activists before people like him. And our country health management isnt reliable, they claim having the second dose after 4 months makes no difference, but they have no study to support their claims and not even the vaccine creators stated they "believe" it will be the same after 4 months. They stated they believe it will be the same up to 3 months.
    – Pablo
    Jun 9 at 15:35

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