Pfizer AstraZeneca COVID vaccine antibody levels may decline in 2-3 months: Lancet Study

Total antibody concentrations start to decline in the sixth week after complete immunisation with Pfizer and AstraZeneca vaccines. A study presented in The Lancet journal on July 15, reported that antibodies wane by more than 50 percent over 10 weeks. In the absence of ongoing antibody synthesis, antibody concentrations decay at a predictable, exponential rate, explained Professor Eleanor Riley, Immunology and Infectious Diseases, University of Edinburgh, and an author of the study.

"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.

The researchers from the University College of London (UCL) studied that antibody levels dropping at the rate could begin to "wear off the protective effects" of the vaccines against the new variants. However, the frequency at which it is to happen is not ascertained in the study. The researchers have also noted that antibody levels are substantially higher following two doses of Pfizer vaccine than after both jabs of the AstraZeneca preventive, which is called Covishield in India, PTI reported.

According to the Lancet study, antibody levels in subjects were initially much higher in vaccinated people than those with before SARS-CoV-2 infection, author and researcher of the study Madhumita Shrotri from UCL Institute of Health Informatics said. However, there was a subsequent drop in the antibody levels over the course of 2-3months, Shrotri added.

For Pfizer, antibody levels reduced from a median of 7,506 Units per millilitre (U/mL) at 21–41 days, to 3,320 U/mL at 70 or more days. For the AstraZeneca vaccine, antibody levels reduced from a median of 1,201 U/mL at 0–20 days to 190 U/mL at 70 or more days, over a five-fold reduction.

'It is not necessarily a problem': Immunologist Prof Riley

Even though there is a subsequent decrease in the antibody levels after approximately 6 weeks of getting vaccinated, the condition "is not necessarily a problem," clarified Professor Riley. She explained the parameters on which the antibody concentrations are measured. She also added that the cells take up a process called 'memory response' in the face of infections. “The two key parameters are first, the minimum concentration of antibodies required for protection and secondly, how quickly antibody concentrations can increase again in the face of infection (the so-called memory response)," she explained.

"The decline in antibody concentrations in the immediate few weeks after vaccination is exactly what I would expect to see," Professor Eleanor Riley.

While talking about the drawbacks of the study, she said, "This study does not answer these questions and, indeed, they are perhaps the most crucial questions we need to answer in order to determine the need for booster doses." As the authors themselves say “the clinical implications of waning antibody levels are not yet clear and it remains crucial to establish S-antibody thresholds associated with protection against infection and disease”.

Lower antibody, stronger T-cells

The findings were based on data collected from over 600 people aged 18 and above, which were consistent across all groups of people regardless of age, chronic illnesses, or sex, according to the researchers. 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.

mRNA vaccines such as the Pfizer/BioNTech vaccine are designed to induce high concentrations of antibodies. Viral vectored vaccines (such as the Oxford-AstraZeneca vaccine) tend to induce lower antibody responses but stronger T cell responses. The differences in antibody concentrations induced by the two vaccines are thus not surprising and not a cause for concern. “However, emerging evidence suggests that antibodies are particularly important for blocking infection and preventing onward transmission of the virus whereas T cells may be particularly relevant for preventing severe disease and death. Maintaining sufficient antibody concentrations to reduce transmission will be important to limit the amount of circulating virus but maybe less important for protection against severe disease," Professor Riley concluded.

(Image input: AP)