Let’s talk about vaccines and dosing again (another thread;) )
There is a lot of talk at the moment about dosing and delays. First, it was the UK who decided to delay the second dose, now more countries need to do that because of the raw material supply issues (1/n)

(which should hopefully be resolved by next week).
Delaying the dose should be still ok, and you should still be able to develop an immune response against SARS-CoV2.(2/n)

I think that the shorter gap between doses was also chosen for the trials because of the lack of time, enabling us to observe the build-up of the antibodies as soon as possible. (3/n)

Because production needed to be lowered for a week to correct manufacturing lines and make the supply higher from Feb-March, many countries faced the problem of not having enough vaccines to give second doses to people already vaccinated on time. (4/n)

In many cases that interval is passing the time recommended by the manufacturer. That’s not ideal but also not a tragedy if we will be sensible.
(5/n)

First of all, you still need your second dose. Even if you pass 28 days between doses you need the second dose of the vaccine. After the first dose, your immune system needs to build up the immune response. (6/n)

That can take up to 14 days so behave as though you are not protected at all during that period. After that time your immune response will be slowly building up and can be between 33% (Israel vaccination early data) to 80% (Pfizer trial data). (7/n)

Why such a difference? Trials are not real life in many ways. Vaccine producers try to get as many people to take part in the trial as possible but of course, there will be limits. (8/n)

Most trial participants also tend to be young and healthy so the immune response in these will also be stronger and better. In ‘real-life’ we started with vaccinating older population and risk groups, which we already know (from clinical trials) have weaker immune response (9/n)

(but 33% is still better protection than 0%). But even if you are young do not think that you are fully protected after the first dose. You still need your second dose and after that, you are protected against severe disease (more than 90% efficacy for all the vaccines). (10/n)

What about the AstraZeneca adenoviral vaccine? Here the recommended gap between doses is much longer - around 56 days. And you might have heard that there was some confusion with dosing in the case of this vaccine. (11/n)

Some people received two full doses, while others got half dose followed by the full dose and surprisingly the latter developed a better immune response. Despite that, the regulatory committee approved the two full dose scheme and not the one which seems to have higher efficacy.

Why is that? Well, it is not really true that a low dose followed by a higher one gave better protection. It was not the dosing. It was the gap between doses. People who got half and full dose had a longer gap between those doses due to lockdown and supply issues (13/n)

Basically, the people who ran the trial needed to pay for a private jet to get the vaccine doses to them and instead of 20-something days like in the other arms of the trial, vaccines were given after more than a month. (14/n)

This was followed by the trial arm which checked how two full doses with a long interval worked and it proved to be more effective than low dose/full dose. And that makes sense. With most of our vaccines, we need to give our immune system time to mature (15/n)

That takes time. Often much longer than 20 days.
Unfortunately in the case of mRNA vaccines, we haven’t yet tested longer intervals so it is hard to say how the dosing will work.(16/n)

But the second dose should also work after a longer period of time - and still needs to be given. The first dose is there to prime your immune system and teaches it to ‘recognise’ the pathogen. The second is given for your immune system to remember it for longer.(17/n)

So please be vigilant, wear your mask as long as it is recommended (so until the sufficient part of the population will be fully vaccinated)and even if delayed, take your second dose.(18/n)

CDC recommendations regarding COVID-19 vaccines:
https://www.cdc.gov/.../clinical-considerations.html...

UK recommendations regarding COVID-19 vaccines
https://www.gov.uk/.../covid-19-the-green-book-chapter-14a

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