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Pfizer, AstraZeneca vaccine antibody levels may decline after 2-3 months: Lancet study

The researchers from University College London (UCL) in the UK noted that if the antibody levels carry on dropping at this rate, there are concerns that the protective effects of the vaccines may also begin to wear off, particularly against new variants.

July 27, 2021 / 12:26 PM IST
Earlier, Pfizer CEO Albert Bourla had also said that the US-based drug maker is in the final stages to sign an agreement with the Indian government to supply COVID-19 vaccines. (Representative image)

Earlier, Pfizer CEO Albert Bourla had also said that the US-based drug maker is in the final stages to sign an agreement with the Indian government to supply COVID-19 vaccines. (Representative image)

Total antibody levels start to wane six weeks after complete immunisation with Pfizer and AstraZeneca vaccines, and can reduce by more than 50 per cent over 10 weeks, according to study published in The Lancet journal.

The researchers from University College London (UCL) in the UK noted that if the antibody levels carry on dropping at this rate, there are concerns that the protective effects of the vaccines may also begin to wear off, particularly against new variants.

However, they said, how soon that might happen cannot be predicted yet.

The UCL Virus Watch study also found that antibody levels are substantially higher following two doses of the Pfizer vaccine than after two shots of the AstraZeneca preventive, known as Covishield in India.

Antibody levels were also much higher in vaccinated people than those with prior SARS-CoV-2 infection, they said.

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COVID-19 Vaccine

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How does a vaccine work?

A vaccine works by mimicking a natural infection. A vaccine not only induces immune response to protect people from any future COVID-19 infection, but also helps quickly build herd immunity to put an end to the pandemic. Herd immunity occurs when a sufficient percentage of a population becomes immune to a disease, making the spread of disease from person to person unlikely. The good news is that SARS-CoV-2 virus has been fairly stable, which increases the viability of a vaccine.

How many types of vaccines are there?

There are broadly four types of vaccine — one, a vaccine based on the whole virus (this could be either inactivated, or an attenuated [weakened] virus vaccine); two, a non-replicating viral vector vaccine that uses a benign virus as vector that carries the antigen of SARS-CoV; three, nucleic-acid vaccines that have genetic material like DNA and RNA of antigens like spike protein given to a person, helping human cells decode genetic material and produce the vaccine; and four, protein subunit vaccine wherein the recombinant proteins of SARS-COV-2 along with an adjuvant (booster) is given as a vaccine.

What does it take to develop a vaccine of this kind?

Vaccine development is a long, complex process. Unlike drugs that are given to people with a diseased, vaccines are given to healthy people and also vulnerable sections such as children, pregnant women and the elderly. So rigorous tests are compulsory. History says that the fastest time it took to develop a vaccine is five years, but it usually takes double or sometimes triple that time.

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"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 Madhumita Shrotri from UCL Institute of Health Informatics.

"However, we found these levels dropped substantially over the course of two to three months," Shrotri said in a statement.

The findings based on data from over 600 people aged 18 and above were consistent across all groups of people regardless of age, chronic illnesses or sex, according to the researchers.

The authors highlight that although the clinical implications of waning antibody levels are not yet clear, some decline was expected and current research shows that vaccines remain effective against severe disease.

For Pfizer, antibody levels reduced from a median of 7506 Units per millilitre (U/mL) at 21–41 days, to 3320 U/mL at 70 or more days.

For AstraZeneca vaccine, antibody levels reduced from a median of 1201 U/mL at 0–20 days to 190 U/mL at 70 or more days, over five-fold reduction.

"When we are thinking about who should be prioritised for booster doses our data suggests that those vaccinated earliest, particularly with the AstraZeneca vaccine, are likely to now have the lowest antibody levels," said Professor Rob Aldridge from UCL Institute of Health Informatics.

The findings support recommendations that adults who are clinically vulnerable, those aged 70 years or over, and all residents of care homes for older adults should be prioritised for booster doses, the researchers said.

In addition, those who were vaccinated with the AstraZeneca vaccine are likely to have much lower antibody levels than those vaccinated with the Pfizer vaccine, they noted.

"This may also need to be considered when deciding who should be prioritised when boosters are rolled out," Aldridge said in the statement.

The team acknowledged certain limitations in the data, including a small sample size for some groups.

The researchers noted that 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 neutralising ability of their antibodies as well as their T-cell responses.

"Even when measurable antibody levels are low, there is likely to be continuing immune memory that could offer long-term protection," the authors of the study noted.

They said further research will be important to establish if there is an antibody level threshold needed for protection against severe disease.
PTI
first published: Jul 27, 2021 12:27 pm

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