British drugmaker AstraZeneca has admitted to a manufacturing error which accidentally led to a highly promising result.

After disclosing promising results from its Covid-19 vaccine trial earlier this week, AstraZeneca has now admitted to a manufacturing error by a contractor. The error came to light after AstraZeneca, which is developing the potential Covid-19 vaccine in partnership with Oxford University, revealed different efficacy results from two different dose regimes.

But that is only half of the story. As result of the error only half of the proposed dose of vaccine was administered to a group of people, while the other group was given the full dose.

In the two different dose regimens vaccine efficacy was 90 per cent in one and 62 per cent in the other.

The higher efficacy regime used a halved first dose and standard second dose. Two full doses showed lower efficacy.

In other words, said the biotechnology company, in the low-dose group, the vaccine appeared to be 90 per cent effective. In the group that got two full doses, the vaccine appeared to be 62 per cent effective. Combined, the drugmakers said, the vaccine appeared to be 70 per cent effective.

But the way in which the results were arrived at and reported by the companies has led to pointed questions from experts. In India, this vaccine is being manufactured within the country by the Serum Institute of India in Pune.

AstraZeneca made a key “mistake” in the vaccine dosage received by some volunteers only after disclosing the preliminary results, according to a report in the New York Times on Wednesday.

This raised questions about whether the vaccine’s efficacy rate derived from the interim analysis of the Phase 3 trial will hold up under additional testing, said the report. Around 3,000 people had already been given the shot.

AstraZeneca revealed the interim results after Pfizer and Moderna had earlier reported late-stage data for their Covid-19 vaccine candidates.

The company said that it reported the positive results from an interim analysis of clinical trials of the “AZD1222” vaccine in the UK and Brazil.

It further added that it continues to engage with Governments, multilateral organisations and collaborators around the world to ensure broad and equitable access to the vaccine at no profit for the duration of the pandemic.