*Jacqui Wise, UK London, DOI: 10.1136/bmj.n326

While developed countries such as Germany, the Netherlands, and the United States have a surplus of covid vaccines and are looking for ways to get citizen vaccinated before the drug expires, in developing countries like Vietnam and Thailand, the government has to struggle to find vaccines from many different sources and then, people don't have much of chance to choose the time between two injections: so what's the difference between the second shot at 6 weeks and 12 weeks apart?

In the UK, the interval between two doses of Oxford Astrazeneca vaccine is 12 weeks. The vaccine "could have a significant impact on community transmission of the virus," say researchers at the University of Oxford.

Based on a scientific paper by Voysey M et al [1], an analysis was conducted on 17,177 participants in the UK, Brazil and South Africa. Actual data was collected for one month, of which 332 had symptoms of covid-19, adding 201 symptomatic cases in the previous study [2]. The research papers are being reviewed in the Lancet (reputable medical journal in the world).

A standard dose of vaccine provides 76% protection against symptoms of covid when acquired 90 days after vaccination. However, it remains unclear how long the protection may last with a single dose as there are too few cases after 90 days to make any meaningful assessment.

The study found that vaccine efficacy was 82.4% after the second dose in those with vaccination intervals of 12 weeks or more (95% CI 62.7% to 91.7%). If the two doses were given less than six weeks apart, the efficacy was only 54.9% (CI 32.7% to 69.7%).

In studies in the UK, scientists took weekly nasal and throat swabs from volunteers for epidemiological testing. The vaccine proved to be quite poor at preventing asymptomatic infection; Its effectiveness increases with longer dosing intervals and wider confidence intervals (more precise data).

However, the researchers found that after one dose of vaccination, polymerase chain reaction test results on covid-19 positive cases (both symptomatic and asymptomatic) were reduced by 67 %, raising hopes of a significant impact on containing infection by reducing the number of infected people in the population.

Andrew Pollard, principal investigator of the Oxford vaccine trial, and co-author of the paper, said the data would "support government policies to increase the interval between two doses to 12 weeks", and asserts that people are protected 22 days after a dose of the vaccine."

Commenting on the study, Paul Hunter, professor of medicine at the University of East Anglia, said, “Given this evidence, a 12-week interval between the first and second doses is clearly the better strategy because many people can be protected faster and the final protection effect is greater. Because of its poor effectiveness in preventing asymptomatic infection, the vaccine will not stop covid transmission, but it still goes a long way to reducing transmission, because there will be fewer infections. more symptomatic, and those with symptoms are more contagious than those who are asymptomatic."

Azra Ghani, chair of infectious disease epidemiology, at Imperial College London, cautions against the results, pointing out that the study was not designed to look at dose intervals or one-to-two. dose. “Participants who received a single dose were younger, more likely to be female, more likely to be healthcare workers, more likely to reside in Brazil, and more likely to be Caucasian than those receiving the injection. two doses. In addition, those who received a single dose were followed for a significantly longer time. This means that it is not reasonable to compare efficacy estimates from a single dose versus two doses.”

[1] Voysey M, Clemens S, Madhi S, et al. Single dose administration, and the influence of the timing of the booster dose on immunogenicity and efficacy of ChAdOx1nCoV-19 (AZD1222) vaccine. https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3777268

[2] Voysey M, Clemens S, Madhi S, et al. Safety and efficacy of the ChAdOx1 nCoV-19 vaccine (AZD1222) against SARS-CoV-2: an interim analysis of four randomised controlled trials in Brazil, South Africa, and the UK. Lancet. 2021. Jan 9; 397(10269):99-111