How Many Antibodies Does It Take to Be Protected from the Virus after Vaccination?

How many antibodies does it take to be protected from Covid-19 after vaccination? Researchers at Oxford University believe they have found the answers. AstraZeneca COVID-19 Vaccine

Covid vaccines protect us because they stimulate the production of specific antibodies against the coronavirus that prevent the virus from completing its viral cycle and multiplying in our cells. But what the scientific community is struggling to determine is the actual number of antibodies needed at which a vaccinated person is protected.

A research group at Oxford University has tackled this question and published their results in medRxiv at the end of June. The researchers found a correlation between antibody levels and the efficacy of AstraZeneca’s vaccine.

To achieve 80% efficacy against symptomatic forms of Covid, an anti-S-protein IgG level of 40,923 (absolute value), an anti-RBD IgG level of 63,383 (absolute value), and a neutralizing antibody level between 180 and 247 (absolute value) are required.

Antibody levels and vaccine efficacy

The study used data obtained from volunteers in a U.K. clinical trial of AstraZeneca’s vaccine. Serum from volunteers vaccinated with Covid-19 – both symptomatic and non-symptomatic – was collected 28 days after the second dose. The researchers quantified three types of antibodies: anti-S-protein IgG, anti-RBD IgG, and neutralizing antibodies.

They established a correlation between the number of antibodies and the efficacy of the AstraZeneca vaccine, and thus the risk of getting sick. To achieve 80% efficacy against symptomatic forms of Covid, an anti-S-protein IgG level of 40,923 (absolute value), an anti-RBD IgG level of 63,383 (absolute value), and a neutralizing antibody level between 180 and 247 (absolute value) are required. This is one of the first studies to demonstrate this correlation.

A correlation to be taken with caution

Does this mean that if the levels of antibodies are under these numbers, the vaccine did not work? This is a tempting interpretation, but these results are not without flaws. First, they refer only to the AstraZeneca vaccine, which is not the most widely used vaccine in the US, and are not necessarily transferable to other available formulations. In addition, the confidence intervals of the antibody levels are very wide. In other words, there is a lot of variability between individuals.

If the antibody levels are lower than those indicated here, it does not necessarily mean that the vaccine did not work. Nor do higher levels mean that someone is overprotected and should forgo all other precautions.

NB: Because AstraZeneca, Johnson & Johnson, and Sputnik V are all COVID-19 adenovirus vector vaccines it would not be far-fetched to assume that the results of the study may apply to all of them.