AstraZeneca plc’s covid-19 vaccine, the company said that the jab showed an efficacy of 79 per cent in phase 3 trials in the US, Chile and Peru, much higher than the results from the UK trial announced in November.

Interestingly, the US trial data showed that the vaccine’s efficacy of 79 per cent was achieved when two standard doses were given at an interval of four weeks, while data from UK trials showed an efficacy of 53 per cent for intervals of less than six weeks. In the UK and Brazil, the efficacy was an average of 62 per cent. Due to some errors by Oxford researchers in the US, the actual efficacy varied between 53 per cent and 90 per cent, depending on the amount of dosage given and the interval between two doses. The interval varied between four weeks and 12 weeks.

In the US trial, the analysis was based on 32,449 participants, including 141 symptomatic cases of COVID-19. The participants were randomized 2:1 with twice as many participants getting the vaccine as compared to the placebo group.

The Anglo-Swedish drugmaker said the result was consistent across ethnicity and age with vaccine efficacy being 80 per cent among participants aged 65 years and over. The trials in UK had limited efficacy data for the elderly due to the small number of cases.

“These findings reconfirm previous results observed in AZD1222 trials across all adult populations but it’s exciting to see similar efficacy results in people over 65 for the first time,” said Ann Falsey, professor of medicine at the University of Rochester School of Medicine and co-lead principal investigator for the trial.