Covid-19 and vaccines – a (retired) doctor writes…

Not only is Andrew Polmear our intrepid wine and cheese man, he’s also a retired doctor and asks… Does delaying the second dose till 12 weeks after the first put us at risk?

One of the joys of being a retired doctor is having the satisfaction of responding to people’s concerns about medical matters without the worry of being responsible for them. So I enjoy queries from friends; and of all the questions I’ve been asked about Covid-19, the commonest is about the 12 week delay between jabs. Is it safe?

The short answer is yes, but, of course, it’s a bit more complicated than that.

The first bit of the answer is the straightforward one that Matt Hancock, Health Secretary, gave when he announced the change. If you have a vaccine that’s in short supply it’s better to give everyone at risk a first dose rather than give some a second dose and others nothing at this stage. The first dose gives a better “bang for your buck”. That means Matt Hancock made the right choice for the country; but it doesn’t stop us, as individuals, wanting that second dose.

But does delaying that second dose reduce our protection?

Things are easiest to understand with the Oxford/AstraZeneca vaccine, because there’s published information about this. The Oxford researchers had intended to test a single dose. But after they’d started the trials they found that, when they gave a second jab to a small group, they got a better antibody response. They went back to the regulator and got permission to change the trial, so they could give a second jab 28 days after the first. However, by the time they gave their subjects the second dose, some were at least 90 days after the first. So they could compare the effect of early and late second jabs:

  • second dose given early (between 22 and 90 days after the first) – 76% of cases of Covid were prevented, with no sign that protection was tailing off towards the end (and none of those who did get Covid were bad enough to go to hospital).
  • second dose given late (more than 90 days) – 82% protection. What’s more, subjects had higher antibody levels than those who had an earlier second dose.

So, with the Oxford/AZ vaccine, we can say that a 90 day gap is fine, possibly even better than a 28 day gap.

With the Pfizer vaccine it’s more difficult because all of those taking part in the trial got the second dose at between 19 and 42 days after the first. So Pfizer are correct to say that there’s no evidence that delaying the second dose until 90 days is safe. The World Health Organisation hedges its bets by recommending that the gap go up to 6 weeks. So why does our own, highly regarded, Joint Committee on Vaccination and Immunisation recommend 3 months? There are two reasons:

  • The first is that most, possibly all, vaccines give better long-term immunity if the second dose is delayed. The HPV vaccine, for instance, given to girls and boys to protect against certain types of cancer, works best if the second dose is given between 6 months and two years after the first. Immunity from vaccination doesn’t fall off rapidly. It takes 2 – 3 weeks after a jab to develop. It doesn’t then suddenly disappear, at least not for months or years.
  • The second reason is a detailed look at the Pfizer trial. Patients in the week before the second jab were 89% protected against Covid-19. After the second jab they were 95% protected. Such a small improvement in protection from an early second dose isn’t statistically significant.

 So, the chances are that a delay in receiving a second dose of the Pfizer vaccine will do no harm. And it’ll certainly mean that more people will get their first jab sooner; which is good for everybody.

This is an area of science where new information is coming out every day. By the time this article is published it may already be out of date. And please bear in mind that it is the opinion of one retired GP, not a leading scientist. Andrew regrets that he cannot enter into correspondence with individual readers.

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