According to a study published in The Lancet journal, total antibody levels begin to decline six weeks after complete immunisation with Pfizer and AstraZeneca vaccines and can drop by more than 50% over ten weeks.
The researchers from University College London (UCL) in the United Kingdom expressed concern that if antibody levels continue to fall at this rate, the vaccines’ protective effects, particularly against new variants, may begin to wear off.
However, they stated that the timing of such an event cannot be predicted at this time.
According to the UCL Virus Watch study, antibody levels are significantly higher after 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 in those who had previously been infected with SARS-CoV-2, according to the researchers.
“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 of the University College London Institute of Health Informatics.
“However, we found these levels dropped substantially over the course of two to three months,” Shrotri said in a statement.
According to the researchers, the findings, which were based on data from over 600 people aged 18 and up, were consistent across all groups of people regardless of age, chronic illnesses, or gender.
Although the clinical implications of waning antibody levels are not yet clear, the authors emphasise that some decline was expected, and current research shows that vaccines remain effective against severe disease.
Pfizer’s antibody levels dropped from a median of 7506 Units per millilitre (U/mL) at 2141 days to 3320 U/mL after 70 days or more.
Antibody levels in the AstraZeneca vaccine decreased from a median of 1201 U/mL at 020 days to 190 U/mL at 70 or more days, 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.
The findings support the researchers’ recommendation that adults who are clinically vulnerable, those aged 70 or older, and all residents of care homes for older adults be prioritised for booster doses.
Furthermore, those who received the AstraZeneca vaccine are likely to have much lower antibody levels than those who received the Pfizer vaccine, according to the researchers.
“This may also need to be considered when deciding who should be prioritised when boosters are rolled out,” Aldridge said in the statement.
The team acknowledged that the data had some limitations, such as a small sample size for some groups.
Because each individual only provided one sample, the researchers cannot yet confirm how quickly antibody levels drop for each individual, or whether they will continue to drop or reach a stable level over the next few months.
They also noted that different people will have varying levels of immunity based on their antibodies’ ability to neutralise viruses as well as their T-cell responses.
“Even when measurable antibody levels are low, there is likely to be continuing immune memory that could offer long-term protection,” the authors of the study noted.
They believe that more research is needed to determine whether an antibody level threshold is required for protection against severe disease.