DELAYING THE SECOND DOSE
All Covid-19 vaccines approved so far are designed to teach the immune system to recognise and defend against the virus with a first dose, and then provide a second booster dose to reinforce that lesson. Some countries are hoping to widen immunisation by giving some protection to as many people as possible with a first dose, and delaying the second doses.
SWITCHING BETWEEN VACCINES
Mixing or switching between Covid-19 vaccines is largely driven by the same aim – vaccinating as many people as possible.
Giving a priming dose of one vaccine and a booster dose of another offers flexibility to offer whichever shots are available.
HAVE THESE STRATEGIES BEEN TESTED IN RIGOROUS TRIALS?
No. Officials have cited limited evidence from trials that the Pfizer-BioNTech, Oxford University-AstraZeneca and Moderna vaccines all confer some protection against Covid-19 after the first dose.
But there is no long-term evidence that any of these vaccines will offer lasting immunity based on just one dose, or how effective they will be if the second dose is delayed.
REDUCING THE AMOUNT OF VACCINE IN EACH DOSE
Health officials in the United States are considering offering half-doses of Moderna’s vaccine to individuals aged 18 to 55. There is some clinical trial data backing this strategy.
Dr Moncef Slaoui, chief adviser to the US vaccine programme, said evidence from a Moderna trial showed that the half-dose induced an “identical immune response” to the higher 100 microgram dose in adults aged 55 and under.
While there is no scientific evidence on the impact of delaying Covid-19 vaccine doses, some experts believe the potential payoff in protecting a larger swathe of the population may be worth it.
Others worry about safety. “There’s just no data,” said virology professor Ian Jones from Britain’s University of Reading.