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Coronavirus antibody levels start declining from as early as six weeks after complete vaccination with Pfizer and AstraZeneca jabs and can reduce by more than half over 10 weeks, pointing to the need for booster doses, according to University College London (UCL) researchers.

The findings, published as a research letter in The Lancet, are based on data from over 600 people. They also show that antibody levels are substantially higher following two doses of the Pfizer or AstraZeneca vaccine, manufactured in India as Covishield by Serum Institute of India (SII). The antibodies are also much higher in those who have received both jabs after becoming infected with covid-19.

India is also in talks to include the Pfizer vaccine in the nationwide immunization programme.

The research highlights that although the clinical implications of waning antibody levels are not yet clear, some decline was expected, and, to be sure, current research shows vaccines remain effective against severe disease. These findings were consistent across all groups of people regardless of age, chronic illnesses or sex.

For Pfizer, antibody levels reduced from a median of 7,506 U/mL at 21–41 days to 3,320 U/mL at 70 or more days. For AstraZeneca, they came down from a median of 1,201 U/mL at 0–20 days to 190 U/mL (67–644) at 70 or more days. U/mL is units per millimetre.

“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 Dr Maddie Shrotri of UCL Institute of Health Informatics.

“However, we found these levels dropped substantially over the course of 2 -3 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 variants; but we cannot yet predict how soon that might happen," Shrotri said.

The researchers said their data suggests those who were vaccinated the earliest, particularly with the AstraZeneca vaccine, are likely to now have the lowest antibody levels and recommend booster doses for the vulnerable.“Our findings support recommendations from the (UK’s) Joint Committee on Vaccination and Immunisation (JCVI) that adults who are clinically vulnerable, those aged 70 years or over, and all residents of care homes for older adults should be prioritized 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," professor Rob Aldridge of UCL Institute of Health Informatics said.

The authors, however, also noted that limitations of the data include a small sample size for some groups and the fact that each individual only contributed one sample, so they cannot yet confirm how quickly antibody levels drop for each individual or whether these would continue to drop or reach a stable level over the next few months.

They also noted that different people will have different levels of immunity depending on the virus-neutralizing ability of their antibodies as well as their T-cell responses, and even when measurable antibody levels are low, there is likely to be continuing immune memory that could offer long-term protection.

Further research will be important to establish if there is an antibody level threshold needed for protection against severe disease. The authors will continue to collect and analyse data from a larger number of participants, following antibody levels in individuals over a longer period of time, the authors said.

“We know vaccines are excellent at preventing severe disease, but they are not perfect, and our data suggest protection will wane over time," said Prof Andrew Hayward of UCL’s epidemiology and healthcare unit.

“With rapidly increasing covid-19 rates and predictions of very high rates over the next weeks and months, it is important that those who are clinically vulnerable, whose antibody levels may now have dropped, continue to be cautious," he said.

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