Third AstraZeneca shot boosts immunity but not a priority, experts say

A third dose of the AstraZeneca vaccine increased antibody levels of volunteers who received their second dose late last year, according to a draft study by Oxford University researchers.

The study found the immune response among study participants was higher compared to 28 days after they had their second dose.

The results show that the AstraZeneca vaccine could be used as a booster shot to increase immunity, if and when it will be needed. Researchers said more information is needed before deciding whether a third dose should be recommended.

Getting a first dose to more people — such as those in Bangladesh, which is currently experiencing a surge in coronavirus cases — is the current priority, said Andrew Pollard, director at the Oxford Vaccine Group, during a news briefing.

“At this point, with high levels of protection in the U.K. population and no evidence of that being lost, to give third doses now … in the U.K. while other countries have zero doses is not acceptable. We really have to make sure that other countries are protected,” he added.

While immunity wanes over time after a first or second dose, some protection remains, Pollard said.

“Even if we meet the virus some months later, the immune system will remember. It will kick in and make stronger immune responses again, and hopefully, that will protect most people from severe disease,” he said.

The benefits of delaying a second dose

The same draft study found individuals who received a second dose of AstraZeneca and the University of Oxford’s COVID-19 vaccine about 44-45 weeks after their initial shot were able to generate higher levels of antibodies compared with individuals who get a second dose within 12 weeks.

Antibody levels gradually declined weeks after the first dose, but an immune response still existed for nearly a year without additional ones.

However, researchers said it’s currently unclear how long a person with only one dose can “safely sit there with still-good levels of protection,” so getting a second dose sooner rather than later remains the better option.

“If you have a lot of transmission in a population, and then if you've got plenty of supply of vaccine, giving two doses seems to make sense, and you wouldn't wait a year for that second dose,” said Pollard.

“But if you have insufficient supply, even having one dose will have a huge impact on reducing hospitalizations and deaths. And so the priority will be to get the first dose to as many people as possible.”

The study provides important evidence on how an extended gap between doses affects antibody levels and could help inform national immunization strategies. Vaccines are currently in short supply, and some countries are expected to see rollout delays beyond the currently recommended dose interval of four to 12 weeks.

Studying new vaccines

Researchers will be studying the effect of a new COVID-19 vaccine against the beta variant first detected in South Africa.

A phase II/III trial of the new vaccine started over the weekend, aiming to include 2,250 people across Brazil, Poland, South Africa, and the U.K. Individuals who received two shots of either the original AstraZeneca vaccine or a messenger RNA vaccine will get the new vaccine at least three months after their most recent dose.

Unvaccinated individuals will either receive a first dose of the original AstraZeneca vaccine followed in four weeks by a dose of the new vaccine or they will receive two doses of the new vaccine delivered 4-12 weeks apart.

This will be compared with the effectiveness of a third shot of the original AstraZeneca vaccine — a preparedness measure in case the coming months prompt the need for an additional shot.

Pollard said “reassuring data” from real-world observation suggests that the original AstraZeneca vaccine works well against the delta variant first detected in India. As such, he said, the variant is not a top priority for new vaccines.