Otherwise healthy 16 and 17-year-olds will be offered a second dose of the Covid vaccine, following advice from the UK’s independent Joint Committee on Vaccination and Immunisation (JCVI).

In August, the committee recommended these young people receive a single jab, promising to issue further guidance on the timing of a second shot in due course.

Now we have that advice and the scientists are clear: “A second vaccine dose increases the level of protection [against Covid] and is important for extending the duration of protection.”

But as regular FactCheck readers will know, when we’re talking about vaccines, it’s not just their efficacy against disease that’s important – we also need to know about side effects.

And it’s on this front that we find a surprising piece of information buried in today’s announcement: that the second shot seems to be safer for teenagers in the UK than in some other countries, including the US.

That’s because, even though young Brits are being offered exactly the same product as elsewhere – either the Pfizer or the Moderna vaccine – the way they are administered here seems to reduce the chances of extremely rare side effects linked to the jabs.

Myocarditis and pericarditis – inflammation of the heart muscle or membrane – appear more prevalent in young males who receive one of these mRNA vaccines. (We looked at this in more detail in previous FactChecks).

While the committee stressed that “most people recovered quickly” from the conditions, the risk of these possible side effects is partly why it held off on recommending 16 and 17-year-olds receive a second dose in the summer.

In August, the JCVI wrote: “Data from the US suggests that, in males aged 12 to 17 years, 9.8 cases of myocarditis were reported per million first doses given. This rises to 67 per million after the second dose.” That’s a nearly sevenfold increase in risk following dose two.

But the suggestion that the second jab was more dangerous than the first now seems to have been given an important caveat.

The JCVI wrote today: “In reports originating from outside the UK, extremely rare adverse reactions, such as myocarditis (inflammation of the heart) have been reported more frequently after the second vaccine dose compared to the first vaccine dose.”

“However,” the scientists continued, “in countries such as Canada and the UK, who have a longer interval between the first and second doses, rates following the second vaccine dose are closer to the reporting rate after the first dose.”

In other words, it looks like it’s not the second dose itself that makes the difference, but the time between the shots.

In countries such as the US where the jabs are given three or four weeks apart, the side effects remain rare, but there’s a significant rise in reported heart inflammation after the second dose. Yet over here, where the gap is 12 weeks, there appears no such increase.

JCVI data out today shows that among 18 to 29-year-old Brits, there are 21 cases of suspected heart inflammation reported for every million first doses administered. Whereas in other nations, we’d expect the figure to increase after the second dose, in the UK it’s almost identical after dose two, with 20 cases reported for every million second shots.

There are some caveats to this finding, and the JCVI is cautious in how it describes the stats.

Most importantly, “there is currently insufficient data to calculate a precise estimate of the reporting rate post-dose 2 in under 18-year-olds in the UK due to, as yet, relatively limited exposure of dose 2 in these individuals.” In short, not enough kids have had a second dose in the UK yet to make definitive claims about how many might suffer heart inflammation following two doses.

That said, the scientists also note that “available data indicates the reporting rate in under 18-year-olds is not higher than that in the 18 to 29 year age group.” In other words, cases of myocarditis and pericarditis in kids don’t seem to be any higher than in young adults.

The implication is that we can therefore extrapolate the findings from this older age group – that there’s no increased risk with a second dose, so long as it comes 12 weeks after the first – when administered to 16 and 17-year-olds.