Adults under 30 are to be offered a jab other than the one made by AstraZeneca after a new recommendation from the UK’s Joint Committee on Vaccinations and Immunisations (JCVI).

It is “preferable for adults aged under 30 with no underlying conditions to be offered an alternative to the AstraZeneca vaccine where available”, the JCVI said yesterday.

The announcement came after the independent medicines regulator, the MHRA, analysed cases of blood clots among a tiny number of people who had received the jab.

England’s deputy chief medical officer, Professor Jonathan Van Tam, described the incidents – which may or may not have been caused by the vaccine – as “sadly quite serious” but “vanishingly rare”.

Let’s take a look at what we know.

How many cases?

Up to 31 March, the MHRA had received 79 reports of blood clots “alongside low levels of platelets” in people who had been given the AstraZeneca jab. Of those cases, 19 people have died.

We know that at least 18.1 million doses of that particular vaccine had been given by then, according to official figures that take us up to 24 March.

What is the overall risk?

Professor Sir David Spiegelhalter of the Winton Centre for Risk at the University of Cambridge crunched the numbers for the regulator.

His team found that if 100,000 people in their twenties take the AstraZeneca jab, 1.1 might be expected to suffer the blood clot in question.

That risk is lower for those in their thirties (0.8 per 100,000) and declines further as we go up the age groups.

For those in their sixties, the risk of this type of clot following vaccination is 0.2 in 100,000.

How does the risk compare to Covid-19?

All medications carry some risk. The question is whether the risks outweigh the potential benefit.

The Cambridge team found when Covid-19 rates in the community are fairly low – as they are in the UK right now – the benefits of the vaccine outweigh the risks in all age groups, except those aged 20 to 29.

That’s partly because there seems to be an elevated risk of these clots in younger people.

But the main reason is that people of that age are less likely to get seriously ill from Covid-19 – so the direct benefit each individual twenty-something gets from the vaccine is lower.

With coronavirus rates at their current level, vaccinating 100,000 twenty-somethings will spare 0.8 of them from ICU.

Do the same with 30 to 39-year-olds and the benefit in terms of intensive care admissions avoided is much larger at 2.7.

The calculation for the under-30s is very finely balanced – 1.1 potential harms per 100,000 vaccinated versus 0.8 potential benefits.

And it doesn’t take much to tip the scales.

If coronavirus in the community reaches the levels it was at in February this year, the benefits even for the youngest group significantly outweigh the risks – 1.1 potential harms per 100,000 people vaccinated versus 2.2 ICU admissions averted.

Are women at increased risk?

This question was put to Professor Sir Munir Pirmohamed, chair of the Commission on Human Medicines, at yesterday’s MHRA briefing.

The regulator’s data shows of the 79 clotting cases, 51 occurred in women.

Professor Pirmohamed said the “numbers are quite small and that may reflect actually who’s getting the vaccine, because many of the healthcare workers, for example, are women” (health and social care staff have been prioritised so far in the rollout).

He said that if you look at the rates of clotting according to the number of vaccines administered to each sex, “there’s actually no difference between men and women”.

Professor Pirmohamed said “we will need to continue to monitor this”, but “at the moment, we do not have any evidence to say that either men or women are more likely to get this”.

How do the risks compare with other medications?

There’s been a lot of talk about the contraceptive pill, which is prescribed to millions of women in the UK every year, despite also carrying a blood clot risk.

Writing in the Lancet last year, Dr Margaret McCartney points out that “the estimated incidence of a blood clot with the combined oral contraceptive pill is about five per 10,000 women per year” which means that “with 3 million women in the UK or so prescribed the pill each year, we expect more than 300 women to have an iatrogenic harm, a pulmonary embolus.”

On the basis of this and the current evidence on vaccines, Professor Spiegelhalter estimates the risk of a blood clot on the pill is about the same in a single week as the one-off risk of the AstraZeneca jab. (And it’s worth saying that the link between the vaccine and these clots has not yet been definitively proven.)

Are under-30s banned from taking the AstraZeneca jab?

It’s important to stress that the UK authorities are not preventing anyone over 18 from getting the vaccine.

The MHRA says it “is not recommending age restrictions in COVID-19 Vaccine AstraZeneca vaccine use”.

The JCVI’s view is that it is “preferable” for alternatives to be offered “where possible” to otherwise healthy young adults.

The Committee also adds: “All those who have received a first dose of the AstraZeneca vaccine should continue to be offered a second dose of the same vaccine, irrespective of age.”

What are the symptoms to look out for?

As a “precautionary measure”, the MHRA says “anyone who has symptoms four days or more after vaccination is advised to seek prompt medical advice” for symptoms “such as”:

• “a new onset of severe or persistent headache, blurred vision, confusion or seizures”,
• “develop shortness of breath, chest pain, leg swelling or persistent abdominal pain”,
• “unusual skin bruising or pinpoint round spots beyond the injection site”.

Though the regulator stresses that the clots are “extremely rare and unlikely to occur”.