This is why you need that second COVID-19 vaccine dose, says WHO’s Chief Scientist

Hospital admissions have been dramatically reduced by just one dose of the COVID-19 vaccine, public health officials in Scotland have said. Measuring the impact of 1.14 million doses administered between 8 December and 15 February, vaccines from Pfizer and AstraZeneca led to an 85% and 94% drop in hospitalization, respectively.

Most available vaccines, and those in development, require patients to receive two doses. In the latest of its "Science in 5" video series, the World Health Organization’s (WHO) Chief Scientist, Dr Soumya Swaminathan, explained why getting that second shot is important, as well as how much time should elapse between the doses.

“The interval between the doses depends on which vaccine you’re getting and the local authorities, the government, would have made a guideline and would inform you about when the second dose is due,” Dr Swaminathan says.

Mind the gap

The UK is one of the countries that has made substantial progress with its vaccine rollout programme, with more than 16 million people there having received at least one dose. A study submitted to the Lancet concluded that a three-month interval boosted its efficacy. A gap of less than six weeks between doses gave 54.9% protection against the illness. But when that gap was extended to 12 weeks or more, protection rose to 82.4%, according to the British medical journal.

The first dose works by introducing a new antigen to the immune system. In effect, it gives the body a sense of what’s needed from the immune response. “And the second dose is the one that really gives a boost to the immune system,” Dr Swaminathan says, explaining that the antibodies and T cells develop a memory response, “which then lasts for a long time”.

But while getting two full doses is recommended by WHO, there is no evidence that it is a good idea for those doses to come from two different vaccines. There’s also no evidence to suggest that it might be a bad idea, and so WHO appears to be advocating caution. “At the present time,” Dr Swaminathan says, “there isn’t enough data for us to recommend this type of interchangeable two-dose schedule… For the time being, it is recommended to have the second dose with the same vaccine as you had the first dose.”

Safety at speed

The development of vaccines in response to the pandemic has led to greatly shortened production times. This is another point addressed by Dr Swaminathan in the latest WHO video.

In excess of 150 million vaccine doses have been administered around the world, to date. And although the time from lab-to-jab has been much shorter than in any other vaccine, “so far, the safety signals have been reassuring”, she says.

The WHO has a drug safety monitoring system in place, in partnership with pharmaceutical companies and regulators, that tracks all the data relating to the vaccine rollout – particularly in connection with any adverse reactions.

“There hasn’t been anything untoward that’s happened with any of the vaccines that have been really rolled out at a large scale,” Dr Swaminathan concludes. “However, we will continue to watch this very carefully and if there is any evidence of a relationship between a vaccine and a side effect, then that will be analyzed and the guidance to countries will be updated from time to time.”