Health director-general Dr Hisham Abdullah said any delay in getting the second dose would probably delay optimal antibody production.

“Based on the observation in clinical research, the efficacy of the Pfizer-BioNTech vaccine will show from the 12th day after the dispensing of the first dose and the efficacy level will reach up to 89% between the 14th and 21st day, which is the ideal time for the second dose to be dispensed,” he said in a statement here today.

He said, at present, the long-term efficacy of the first dose of the Pfizer-BioNTech vaccine remained unknown as the vaccine recipients in the clinical research were given the two doses within a period of 19 to 42 days.

Meanwhile, Noor Hisham said in the third phase of the clinical research involving two doses of the vaccine given to recipients within a period of 21 days, the efficacy level of the Pfizer-BioNTech vaccine was found to have reached 94.6% in preventing symptomatic Covid-19 infection.

However, the World Health Organization’s Strategic Advisory Group of Experts on Immunization recommended that the second dose be given 21 to 28 days after the first dose, he said.

Noor Hisham added that the health ministry was constantly monitoring the vaccine used in the National Covid-19 Immunisation Programme to ensure its quality, efficacy and safety.