The study led by a team of researchers from University Hospital Southampton in the UK, showed that a booster dose of AstraZeneca, Pfizer, Novavax, Janssen, Moderna, and Curevac is safe and generates good immune response and has less side-effects. Reactions to all seven vaccines were similar, with fatigue, headache, and injection site pain most often reported.
However, increases in anti-spike protein antibody levels after 28 days varied across the vaccines. After two doses of AstraZeneca these ranged from 1.8 times higher to 32.3 times higher according to the booster vaccine used. After two doses of Pfizer the range was 1.3 times higher to 11.5 times higher. Significant T-cell responses were reported in several combinations.
At 28 days, all booster results were similar for participants aged 30-69 years and those aged 70 years or older. However, the researchers warned that the boost ratios should be interpreted with caution because they relate to immunogenicity rather than protection against disease, and the relationship between antibody levels at day 28 and long-term protection and immunological memory is unknown.
"The side effect data show all seven vaccines are safe to use as third doses, with acceptable levels of inflammatory side effects like injection site pain, muscle soreness, fatigue, whilst all boosted spike protein immunogenicity after two doses," said Professor Saul Faust, trial lead and Director of the NIHR Clinical Research Facility, University Hospital Southampton NHS Foundation Trust.
"It's really encouraging that a wide range of vaccines, using different technologies, show benefits as a third dose to either AstraZeneca or Pfizer-BioNTech. That gives confidence and flexibility in developing booster programmes here in the UK and globally, with other factors like supply chain and logistics also in play," Faust added.
A randomised, phase 2 trial of seven booster vaccines was conducted, with the third doses given 10-12 weeks after initial two-dose courses of AstraZeneca or Pfizer. The trial involved 2,878 participants in good health recruited at 18 UK sites between June 1-30, 2021.
Participants had received their first doses of AstraZeneca or Pfizer in December 2020, January or February 2021, and second doses at least 70 days before enrolment for AstraZeneca and at least 84 days for Pfizer. Around half of participants received two doses of AstraZeneca and half two doses of Pfizer. The control vaccine used was a meningococcal conjugate vaccine.
Thirteen experimental and control arms of the trial (seven vaccines plus three at half dose and three control arms) were split into three participant groups, with six sites per group.
These were more commonly reported by those aged 30-69. 912 of the 2,878 participants experienced a total of 1,036 adverse events -- effects beyond these side effects, 24 of which were severe.
The study also acknowledged several limitations such as pandemic timelines and the need to generate data to inform policy in September 2021, the interval between second and third doses was shorter in some participants than between their first two doses.