The decline in antibody concentrations in the immediate few weeks after vaccination is exactly what I would expect to see. In the absence of ongoing antibody synthesis, antibody concentrations decay at a predictable, exponential rate. This is not necessarily a problem.

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. 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”.

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.

‍