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According to a new study, total antibody produced by Pfizer and AstraZeneca have begun to decline within six weeks after two vaccine doses, and are to drop by more than 50% in ten weeks, highlighting the importance of booster doses.

The study, led by University College London researchers, found that the declining effect of vaccine doses was consistent across all age groups, chronic conditions, and sex groupings.

The study, which was published in The Lancet as a research letter, collected data from over 600 people and found that antibody levels are much greater after two doses of the Pfizer vaccine than after two doses of the AstraZeneca vaccine.

They are also substantially higher in people who have already been infected with SARS-CoV-2.

Although the clinical implications of falling antibody levels are not yet evident, the researchers noted that some decline was predicted, and current research suggests that immunizations are still effective against severe disease.

Pfizer’s antibody levels dropped from 7506 U/mL at 21-41 days to 3320 U/mL after 70 days or longer. Antibody levels for AstraZeneca dropped from a median of 1201 U/mL at 0-20 days to 190 U/mL (67-644) after 70 days or more.

“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 Maddie Shrotri from the UCL Institute of Health Informatics.

“However, we found these levels dropped substantially over the course of two to three months. If they carry on dropping at this rate, we are concerned that the protective effects of the vaccines may also begin to wear off, particularly against new variants; but we cannot yet predict how soon that might happen,” she added.

The findings back up the Joint Committee on Vaccination and Immunization’s (JCVI) recommendation that adults who are clinically susceptible, those aged 70 and up, and all inhabitants of care facilities for the elderly be given priority for booster doses.

“In addition, those who were vaccinated with the AstraZeneca vaccine, including most care home residents, are likely to have much lower antibody levels than those vaccinated with the Pfizer vaccine, so this may also need to be considered when deciding who should be prioritized when boosters are rolled out,” said Professor Rob Aldridge from the UCL Institute of Health Informatics.

However, because the study only contained a small sample size, different persons will have varying levels of immunity based on their antibodies’ ability to neutralize viruses as well as their T-cell responses.

As a result, more research is needed to determine if an antibody level threshold is required for protection against severe disease, according to the researchers.

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