Two doses of the AstraZeneca vaccine administered 44-45 weeks apart generated nearly four times the level of antibodies than when the doses were given 8-12 weeks apart, says a report by the Oxford Vaccine Group, the developers of the vaccine, on Monday.
Antibody levels remained elevated for nearly a year and a third booster dose of the vaccine, given to a subset of volunteers, also significantly boosted antibody levels to twice that after a second dose.“A single dose of ChAdOx1 nCoV-19, with a second dose given after a prolonged period, may, therefore, be an effective strategy when vaccine supplies are scarce in the short term. A third dose results in a further increase in immune responses, including greater neutralisation of variant SARS-CoV-2 viruses, and could be used to increase vaccine efficacy against variants in vulnerable populations,” the authors report in a pre-print publication. This means the study is yet to peer-reviewed.
A group of vaccine volunteers who got their second dose 15-25 weeks after the first, saw average antibody levels at nearly twice those in the 8-12 week interval, suggesting that lengthening the duration between doses appeared to be boosting antibody count.
Thus, average IgG (immunoglobulin G) levels for 8-12, 15-25, and 44-46 weeks were 923, 1860 and 3738 units respectively, when measured 28 days after the second dose. The volunteers chosen were among those who'd been part of the phase 1/2 and phase 2/3 clinical trials.
Covishield, which is the India-made version of the AstraZeneca vaccine, is now the mainstay of India's vaccination programme comprising nearly 88% of the 32 crore doses administered so far. Though the dosage interval of the vaccine was initially designed as between 4-6 weeks, a supply crunch in May, as well United Kingdom data on the vaccine’s efficacy administered 8-12 weeks apart weighed on Indian experts to recommend a 12-16 week interval between two doses of the vaccine.
The latest study also reported reduced common adverse events after the second dose compared to the first. After a first dose, antibody levels peaked in 28 days and after 180 days were nearly half as that of the peak. At 320 days, these were only 30% of the peak levels.
However protection against symptomatic disease is a result of the combined action of vaccine induced antibodies and cellular immunity, or T cell immunity, experts have noted, though it isn't known how much of each is required to guarantee complete protection.
A group of scientists, many affiliated to the Oxford Vaccine Group have previously shown that protection against symptomatic COVID-19 from 22 days after the first dose was around 76% and a second dose administered 12 weeks after the first, improved it to 81%. -The Hindu