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What’s the scientific basis for delaying the Covid vaccine second dose? | World news


UK health officials have decided to delay giving second doses of Covid-19 vaccines and even permit combining doses from different manufacturers, prompting international concern. What is the scientific justification for this decision?

Why has the UK decided to lengthen the gap between the first and second doses of vaccine?
The original plan was to offer priority groups an initial shot of vaccine, followed by a second dose three weeks later. But a rapid increase in the number of Covid-19 cases, combined with the emergence of a more transmissible variant and uncertainty about the supply of vaccine stocks, prompted the Joint Committee on Vaccination and Immunisation (JCVI) to consider other options.

It calculated that it could get a first shot of vaccine into more arms faster if it pushed back administration of the second dose to 12 weeks afterwards – by which time it is hoped that far more doses of vaccine will become available. “This is highly likely to have a greater public health impact in the short term and reduce the number of preventable deaths from Covid-19,” a JCVI statement said.

Isn’t this the same strategy proposed by Tony Blair?
The former prime minister proposed giving as many people as possible a single dose of a Covid vaccine rather than preserving stocks to enable a second jab, which is slightly different to what is now taking place.

The JCVI has stressed that everyone will receive a second jab, it will just come later than originally planned, with a maximum interval of 12 weeks between first and second doses. This second “booster” dose is likely to strengthen and optimise the duration of the immune response, resulting in longer-term protection.

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Why are some scientists concerned about it?
Chiefly because the impact of stretching out the two doses hasn’t been tested in clinical trials. Neither has giving a first dose of the Pfizer/BioNTech vaccine, followed by a second dose of the Oxford/AstraZeneca vaccine, or vice versa – which the JCVI advises only in “exceptional circumstances”, eg if a second dose of the original vaccine isn’t available.

The main risk is that people’s level of immunity falls before they receive their second dose, putting them at risk of Covid-19 – although this risk would still be lower than if they’d received no vaccine, and would be boosted when they eventually received their second shot. Pfizer cautioned that its trial only investigated giving two doses 21 days apart – far less than 12 weeks.

However, a consensus statement by the British Society for Immunologists said that delaying the booster dose by eight or nine weeks was unlikely make much difference in thee longer-term.

What about mixing and matching doses from different manufacturers?
This also hasn’t been tested in clinical trials – although UK and Russian scientists are investigating whether combining shots of the Oxford/AstraZeneca and Sputnik V vaccines could result in better protection than two doses of the same one.

Theoretically, combining doses from different manufacturers should still trigger an effective immune response, since almost all of the Covid-19 vaccines being developed are based on the same viral ‘“spike” protein, even if this is presented to our bodies in slightly different ways.

Could other countries follow the UK?
Possibly. The World Health Organization said there was no scientific evidence for a delay of more than six weeks in administering the second Pfizer/BioNTech vaccine dose, but Canada and Germany are believed to be considering this, and Denmark said it would permit a six-week gap to allow more people to receive a first shot. US health officials have ruled out this possibility, however.

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