Falls in antibodies after vaccination are expected and do not necessarily mean people are more vulnerable to disease, but the researchers are concerned that if the declines persist the effectiveness of the vaccines may diminish.

The UCL Virus Watch study found that antibodies generated by two doses of the Oxford/AstraZeneca and Pfizer/BioNTech vaccines started to wane as early as six weeks after the second shot, in some cases falling more than 50% over 10 weeks.

The researchers stress that both vaccines are extremely effective against Covid, but say the findings support plans for a booster campaign this autumn, particularly for those who were vaccinated early and with the Oxford/AstraZeneca shot.