Kenyans who received the first dose of the AstraZeneca vaccine against Covid-19 last month will get the second in June.
This will be a 12-week interval between the two doses, which is within the recommended guidelines. Kenya had earlier preferred an eight-week gap.
A Lancet research shows that a longer delay of the second dose may improve the long-term immune response. The vaccine works better if the two jabs are administered about three months apart.
In the study, 24,422 participants were recruited and vaccinated between April and December last year. Exploratory analyses showed that vaccine efficacy after a single standard dose was 76 per cent from Day 22 to Day 90.
During this period, protection did not wane and antibody levels were maintained, with minimal waning by the last day. Efficacy was also higher in participants with a longer prime-boost interval.
Among those who received two standard doses, efficacy after the second was 81.3 per cent at 12 weeks or more versus 55.1 per cent at less than six weeks.
"Overall, the value of this study is in providing evidence that a single dose of the Covishield vaccine is highly efficacious in the 90 days after vaccination, a longer prime-boost interval results in higher efficacy, and that protection against symptomatic Covid-19 is maintained despite a longer dosing interval," the study stated.
The observations are supported by immune response data, which shows "binding antibody responses more than two-fold higher after an interval of 12 or more weeks compared with an interval of less than six weeks" in participants aged 18-55 years.
The researchers found that vaccine efficacy more than 14 days after the second dose was 66.7 per cent.
With scarcity of the AstraZeneca vaccine across the world, the research report suggests that "a three-month dose interval might have advantages over a programme with a short dose interval for roll-out of a pandemic vaccine to protect the largest number of individuals in the population as early as possible".
"This supports the relationship we have found between vaccine interval and efficacy in those receiving two standard doses, which is the preferred regimen because there are more data to support its use and because it is simpler to deliver a vaccine program when the same vaccine is given for both doses," said Dr Merryn Voysey, lead statistician at the Oxford Vaccine Group.
Health chief administrative secretary, Dr Mercy Mwangangi, said the ministry had revised the interval between the two doses from eight to 12 weeks, considering the evidence and guidance from the World Health Organisation (WHO).
"We will start administering the second dose in the first week of June. It should be noted that the global shortages are beyond us. There is an international effort to deal with it," she said.
Vaccination protocols do not include the mixing of different brands of vaccines, and therefore Kenyans who received the first dose of AstraZeneca will have to wait for the second dose of the same vaccine.
India, the biggest producer of the vaccine, banned exports due to a spike in cases at home. The country is reporting record numbers of deaths as a brutal second wave of the pandemic overwhelms its fragile healthcare system.
Kenya received 1.02 million doses from the Covax facility and an extra 100,000 doses from the Indian government last month. The government expected to receive 3,564,000 doses.
As of Saturday, 822,651 doses had been administered across the country, including 152,469 healthcare workers, 126,322 teachers and 66,677 security personnel.