Such examples are galore as mass Covid vaccination has entered its sixth month. However, the Indian Council of Medical Research National Institute of Virology (ICMR-NIV) does not currently recommend testing for antibodies post-vaccination. Reasons being the amount of antibodies varies among individuals due to their genetic makeup and owing to the variety of antibody tests used.
Also, if the antibody test after vaccination is negative, it does not mean that the vaccine is not giving any protection—vaccination can stimulate memory cells in the body and still confer protection. Conversely, the presence of antibodies after vaccination should not be used as a licence to be careless as antibody-positive people may still get Covid-19, though not serious forms of it, experts have said.
“Post-vaccination antibody testing is not recommended currently due to the variable sensitivity and specificity of the commercially available antibody assays (tests),” Priya Abraham, director of ICMR-NIV, told TOI.
Also, the commercially available antibody tests cannot differentiate between vaccine-induced antibodies and antibodies induced after natural infection or break-through infection post-vaccination. “Besides, the minimum level (threshold) of protective antibodies in the blood is not precisely defined yet. Many breakthrough infections have been reported post-one or two-dose vaccination despite the presence of antibodies. Most are mild cases and require no hospitalization. Therefore, checking antibodies post-vaccination or natural infection serves no purpose,” said senior virologist Deepak Gadkari, former director of ICMR-NIV.
Antibody tests may also vary from one lab to the other and according to the timing after vaccination. “It takes two or three weeks post-vaccination to develop a robust antibody response. It may vary from person to person,” said infectious diseases expert Parikshit Prayag.
Commercially available ant-ibody tests detect antibodies ag-ainst the nucleo-capsid protein as well as the spike protein. “Nucleocapsid antibodies are generated only after natural infection, while the presence of spike protein antibodies (IgG) could mean past infection or response to the Covishield vaccine,” Prayag said. Covishield is a vaccine incorporating only spike protein and not the entire virus.
“We perform spike protein IgG antibody test, which is both quantitative and qualitative, and indicative of the presence of S1 and S2 subunits of the spike protein of the novel coronavirus. These spike protein IgG antibodies are either seen in blood post-natural infection or post-vaccination with Covishield,” said pathologist Avanti Golwilkar-Mehendale. “The SARS-CoV-2 nucleocapsid IgG antibody test will not, therefore, be detected following Covishield vaccine, but can be detected post-natural infection or maybe after Covaxin shot.”
Commercial kits are availa-ble to determine presence of anti-SARS-CoV2 antibodies, but they don’t detect ‘neutralizing’ or protective antibodies. “Neutralization test requires live infectious virus or pseudo-virus to detect ‘protective’ antibodies. Private labs are unlikely to have such facilities,” Gadkari said.