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A guide to serum virus neutralization assays

The what, how and why of serum virus neutralization assays in COVID-19 vaccine development

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While the level of neutralizing antibody titers needed to protect against SARS-CoV-2 infection in humans remains an open question, the assays to test them will be an essential tool for COVID-19 vaccine developers.

Figure: Serum virus neutralization assay

The assay is performed by first collecting serum from an immunized animal and heat-inactivating it. The serum, which contains neutralizing antibodies, is then serially diluted, and various dilutions are mixed with either live virus or pseudovirus.

For a live virus neutralization assay SARS-CoV-2 is added in; for a pseudovirus neutralization assay, a different virus expressing SARS-CoV-2 antigens such as the spike protein is used.

Cells are added to the mixture and incubated, and scored for cytopathic effect (CPE) -- structural changes caused by viral invasion.

The neutralizing antibody titer is reported as the highest dilution of the serum that can still neutralize the virus.

FDA guidance issued in April outlining eligibility criteria for COVID-19 convalescent plasma donors recommends neutralizing antibody titers of at least 160, and of at least 80 if no alternatives are available.

On Thursday, a group led by scientists at Harvard Medical School’s Beth Israel Deaconess Medical Center reported monkey data that provide another antibody benchmark for vaccine developers to meet or beat.

The data showed that monkeys that had recovered from mild COVID-19 had neutralizing antibody titers in the range of 35-326 in a live virus neutralization assay, and 83-197 in a pseudovirus neutralization assay. Upon rechallenge, the monkeys had reduced viral RNA and fewer symptomatic days, suggesting some level of protection (see “Monkey Rechallenge Data Provide COVID-19 Neutralizing Antibody Benchmark”).

Also this week, several groups reported titers of vaccine-induced antibodies in mice and non-human primates; on May 22, CanSino Biologics Inc. (HKEX:6185) reported Phase I results in The Lancet showing its vaccine candidate induced neutralizing antibody titers in in the range of 5-50.

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