London: In what could be an issue of concern, a study published in The Lancet journal says that the total antibody levels start to wane six weeks after complete immunisation with Pfizer and AstraZeneca vaccines, and can reduce by more than 50 per cent over 10 weeks.
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, researchers from University College London (UCL) in the UK noted. However, they said, how soon that might happen cannot be predicted yet.
The UCL Virus Watch study also found that antibody levels are substantially higher following two doses of the Pfizer vaccine than after two shots of the AstraZeneca preventive, 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,” Madhumita Shrotri from UCL Institute of Health Informatics was quoted as saying in The Free Press Journal.
“However, we found these levels dropped substantially over the course of two to three months,” she said in a statement.
The findings based on data from over 600 people aged 18 and above were consistent across all groups of people regardless of age, chronic illnesses or sex, according to the researchers.
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 reduced from a median of 7506 Units per millilitre (U/mL) at 21–41 days, to 3320 U/mL at 70 or more days.
For the AstraZeneca vaccine, antibody levels reduced from a median of 1201 U/mL at 0–20 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,” Professor Rob Aldridge from UCL Institute of Health Informatics was quoted as saying.
The findings support recommendations that adults who are clinically vulnerable, those aged 70 years or over, and all residents of care homes for older adults should be prioritised for booster doses, the researchers said.