Hundreds of thousands of people have failed to come forward for their second Covid jab, official data shows, as scientists warned that improving uptake among adults is more crucial than moving on to children’s vaccines or booster shots.

Experts have repeatedly emphasised the need to receive both doses of the coronavirus vaccines as the second jab greatly increases protection against Covid.

But figures from Public Health England (PHE) suggest take-up of second doses is levelling off in older age groups, and is lower than for first jabs. The data, which extends to 22 August, also shows take-up of first doses has essentially plateaued in almost all eligible age groups except the very youngest, and falls with age.

While nearly 21.4 million people aged 50 and over in England have had their first dose, just under 19.9 million have had their second – a difference of almost 500,000. That’s despite people aged 50-54 being invited for their first jab since 17 March, more than 23 weeks ago. In December, a 12-week gap between doses was recommended, which was cut to eight weeks for those aged 50 and over in May, a move later expanded to all eligible for the jabs.

Some over-50s may only recently have had their first jab and hence not yet be eligible for their second, but the appearance of reluctance to receive second doses is supported by other data.

According to figures from the UK’s Medicines and Healthcare products Regulatory Agency (MHRA), about 23.9m second doses of the Oxford/AstraZeneca jab had been given in the UK as of 18 August. This is far less than the 24.3m first doses administered by 26 May – 12 weeks earlier – and the 24.5m first doses given by 23 June, eight weeks before. As noted by the Independent, this means between 400,000 and 600,000 Britons eligible for a second Oxford/AstraZeneca jab have yet to come forward.

With the MHRA advising that those under 40 should, if possible, be offered a different Covid jab, the majority of these will be older adults and hence are at greater risk from the virus.

Prof Rowland Kao, an epidemiologist at Edinburgh University, said it was not surprising that uptake of second doses was lower as some people would miss appointments and some may yet come forward for their second jab.

“But there may also be less incentive to get a second dose – we are out of lockdown, which would have incentivised some, and with the message being that we are in the all-clear that impetus may be lessened,” said Kao.

Dr Michael Head, a senior research fellow in global health at the University of Southampton, agreed there were probably many reasons behind the figures. “This may include concerns over side-effects and also factors such as having been away on holiday over August or feeling that the second dose isn’t necessary,” he said.

Head said more needed to be done to tackle concerns over rare side-effects, noting research had shown the risks of blood clots were much higher from Covid infection than from the Oxford/AstraZeneca jab.

“This needs to be communicated to the general public to increase confidence and ensure that uptake ends up as high as it possibly can be,” he said. An increase in the number of cases was expected in September as holidays ended and students returned to school and university, he added.

While debates around booster jabs and vaccinating children have dominated headlines in recent weeks, another scientist said persuading more eligible adults to have first two vaccines was more important.

That echoes comments made in July by Sir Andrew Pollard, the head of the Oxford Vaccine Group, who said it was right that the emphasis was on ensuring high uptake of vaccines in adults to minimise the risk to public health and pressure on the NHS.

While Prof Peter Openshaw, an immunologist from Imperial College London, said it was not an either/or situation, he, too, stressed the importance of a second dose, with waning immunity among the concerns.

“It really is very important indeed that they have the second jab, partly because it greatly enhances the immune response triggered by the vaccine but also it means that it will last very much longer,” he said “One jab clearly is not enough .”

The latest data from the Office for National Statistics suggests 98% of adults who received their first dose say they are likely to have their second, the Department of Health said.