•Results demonstrated that antibody levels were increased after a delayed second dose.
• Researchers said a long interval between the first and second dose does not compromise the immune response after a late second injection.
Delaying for a longer period after receiving the first dose of the AstraZeneca vaccine leads to a higher immune response, a study has shown.
A study by researchers from Oxford University found that a longer delay of as long as 45 weeks between the first and second dose of the Oxford-AstraZeneca vaccine leads to enhanced immune response after the second dose.
The study further found that a third dose given more than six months after the second dose leads to a substantial increase in antibodies and induces a strong immune response against SARS-CoV-2, including variants.
The experts conducted the study among volunteers aged 18 to 55 years who were enrolled in a Phase 1/2 or Phase 2/3 clinical trial of ChAdOx1 nCoV-19 and had received either a single dose or two doses. They were invited back for vaccination.
The aim was to assess the persistence of immunity after a single dose, the immunity after an extended interval between the first and second dose of ChAdOx1 vaccine and the response to a third dose as a late booster.
“When examining the effects of a delay of up to 45 weeks between first and second doses in study participants, results demonstrated that antibody levels were increased after a delayed second dose,” the study reads.
“Additionally, a longer delay between first and second doses may be beneficial, resulting in an increased antibody titre and enhanced immune response after the second dose,” it adds.
The research found that 30 participants who received a late second dose of ChAdOx1 vaccine at 44 weeks after first dose had higher antibody titres when compared with the response 28 days after a second dose.
A titre is a measure of how much the sample can be diluted before the antibodies or antigens can no longer be detected.
This news will be a relief to many countries struggling to administer at least one dose of the vaccine due to global supply shortages.
The initial recommendation was the second dose be issued within 12 weeks. Kenya like any other country has been unable to get enough stock to administer the second dose, a situation that has forced the Health Ministry to pause the administering of first doses.
All vaccines coming in will now be focused on giving the second dose to those who had already received the first, with plans underway to procure Johnson and Johnson and Pfizer vaccines to boost the vaccination campaign.
According to the researchers, a long interval between first and second doses does not compromise the immune response after a late second dose, and a third dose of the vaccine continues to boost antibodies against SARS-CoV-2.
‘This should come as reassuring news to countries with lower supplies of the vaccine, which may be concerned about delays in providing second doses to their populations. There is an excellent response to a second dose, even after a 10 month delay from the first,” Prof Sir Andrew Pollard said.
Pollard is a professor of paediatric infection and immunity and lead investigator of the Oxford University trial of the vaccine.
Associate Professor Teresa Lambe OBE who is also a lead senior author for the studies noted, however,it is not known if booster jabs will be needed due to waning immunity or to augment immunity against variants of concern.
"Here we show that a third dose of ChAdOx1 nCoV-19 is well tolerated and significantly boosts the antibody response. This is very encouraging news, if we find that a third dose is needed,"Lamb said.
The study found the side effects of the vaccine itself to be lower after second and third doses than after first doses.
As at Monday, 289,800 people had received their second jab, with the largest percentage being men at 56 per cent.
Data from the Health Ministry shows 68,688 healthcare workers have taken their second dose while the number of teachers stands at 36,167.
The number of security officers who have received their second dose is 21,781, 84,855 are people aged above 58 years while 78,309 are members of the general public.
Health CS Mutahi Kagwe, 1,003,204 Kenyans have received their first dose of the vaccine.
(Edited by V. Graham)