Antibody levels generated by two doses of the Oxford/AstraZeneca or Pfizer/BioNTech coronavirus vaccine may start to fall six weeks after the second jab, and for some could drop by up to 50 percent in less than three months, research suggests.
University College London’s (UCL) Virus Watch study analysed blood from 552 vaccinated people mostly in their 50s and 60s.
Antibody levels varied widely between patients, but a double dose of Pfizer/BioNTech produced more antibodies against the virus than two Oxford jabs.
For the Pfizer vaccine, antibody levels fell from an average of 7,506 units per millilitre (ml) at 21–41 days, to 3,320 units per ml at 70 or more days.
For the Oxford jab, they fell from 1,201 units per ml at 0–20 days to 190 units per ml at 70 or more days.
The letter said it might be "important if antibody levels in some groups drop below (as yet undefined) thresholds of protection earlier than in others" as higher antibody levels are "possibly associated with greater protection against variants," but also said that T-cell responses can possibly "compensate to some extent as antibody responses wane."
Eleanor Riley, professor of immunology and infectious disease at the University of Edinburgh, said the result is expected and poses no problem.
“In the absence of ongoing antibody synthesis, antibody concentrations decay at a predictable, exponential rate. This is not necessarily a problem," she said.
“The two key parameters are: firstly, 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).
“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.”
She added: “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 response but stronger T cell responses.
The differences in antibody concentrations induced by the two vaccines is 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.”