However, while antibodies are expected to wane this does mean people are less protected against the virus.
It is also not known how quickly the concentrations can increase again in the face of infection, something referred to as memory response.
But in anticipation of immunity waning, the NHS in England is starting to plan a coronavirus booster jab programme from September.
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Officials have stressed that a third vaccine may not be needed, but scientific advisers to the Government have said they are taking no chances in being prepared.
UCL’s Virus Watch study analysed blood from 552 vaccinated people mostly in their 50s and 60s.
For Pfizer vaccine antibody levels fell from an average of units per millilitre (ml) at 21–41 days, to 3320 units per ml at 70 or more days.
For the Oxford jab they fell from 1201 units per ml at 0–20 days to 190 units per ml at 70 or more days.
The findings are published in the results in a research letter to the Lancet.
The authors write: “In the context of recent advice in support of booster vaccinations from the UK’s Joint Committee on Vaccination and Immunisation, and given the potentially rapid S-antibody decline suggested by these data, heterologous regimens, which preliminary data suggest elicit stronger antibody and T-cell responses, might provide more durable immunity and greater protection against emerging variants.
They add: “Principally, the ethical basis for universal booster dose deployment in high income settings should be carefully considered in the context of widening global vaccine inequities.
“Data on disparities in peak antibody levels and rates of decline might therefore inform targeted and equitable booster deployment.”
Eleanor Riley, professor of immunology and infectious disease, University of Edinburgh, said: “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.”
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.”
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