Six different Covid-19 vaccines administered as booster doses are safe and elicit strong immune response among people who earlier received both doses of the AstraZeneca or Pfizer vaccines, according to a study published in the UK medical journal The Lancet.

Two doses of AstraZeneca and Pfizer vaccines have shown 79 percent and 90 percent protection, respectively against hospitalisation and death after six months in several studies, according to researchers.

However, protection against Covid-19 infection wanes over time, which has driven the consideration of boosters to protect the most vulnerable population and lessen pressure on the health infrastructure.

The latest study looked at the efficacy of seven vaccines given after two initial doses of either the Oxford-AstraZeneca or Pfizer-BioNTech vaccine.

The vaccines were AstraZeneca, Pfizer-BioNTech, Novavax, Janssen, Moderna, Valneva, and Curevac.

The study data showed all seven vaccines were safe to use as third doses, with acceptable levels of inflammatory side effects like injection site pain, muscle soreness, fatigue.

Professor Saul Faust, trial lead from the University Hospital Southampton NHS Foundation Trust, UK, said the finding were "really encouraging".