Antibody levels after completing immunisation with Pfizer and AstraZeneca vaccines may be reduced by over 50 per cent after 2-3 months or 10 weeks, a Lancet study has found.
A study published in The Lancet journal has found that the total antibody levels start to become weaker six weeks after completing immunisation with Pfizer and AstraZeneca vaccines. The antibody levels may also be reduced by more than 50 per cent over 10 weeks or 2-3 months.
According to researchers from University College London (UCL) in the UK, if the antibody levels carry on dropping at this rate, there are concerns that the protective effects of the vaccines may also begin to wear off, particularly against new variants. But how soon that might happen cannot be predicted yet, they added.
The UCL Virus Watch study also found that antibody levels are substantially higher following two doses of the Pfizer vaccine compared to two shots of the AstraZeneca jab, known as Covishield in India.
Antibody levels were also much higher in vaccinated people than those with prior SARS-CoV-2 infection, they said.
"The levels of antibody following both doses of either the AstraZeneca or Pfizer vaccine were initially very high, which is likely to be an important part of why they are so protective against severe Covid-19," said Madhumita Shrotri from UCL Institute of Health Informatics in a statement.
However, we found these levels dropped substantially over the course of two to three months," Madhumita Shotri added.
Findings consistent across all groups of people
Based on data from more than 600 people aged 18 and above, the findings of the study were consistent across all groups of people, regardless of age, chronic illnesses or sex, researchers said.
The authors highlighted that although the clinical implications of waning antibody levels are not yet clear, some decline was expected and current research shows that vaccines remain effective against severe disease.
For Pfizer, antibody levels declined from a median of 7506 units per millilitre (U/mL) at 2141 days, to 3320 U/mL on 70 or more days.
For the AstraZeneca vaccine, antibody levels were reduced from a median of 1201 U/mL at 020 days to 190 U/mL at 70 or more days, over a five-fold reduction.
"When we are thinking about who should be prioritised for booster doses, our data suggests that those vaccinated earliest, particularly with the AstraZeneca vaccine, are likely to now have the lowest antibody levels," said Professor Rob Aldridge from UCL Institute of Health Informatics.