Sequential observation of antibody response pattern at 3 and 6 months following ChAdOx1 nCoV-19 vaccination among health care workers-A prospective single cohort study

The duration of the immune response induced by ChAdOx1 nCoV-19 vaccination in a real-world setting is unknown. 

This study is aimed to estimate the 6-month trend of SARS-CoV-2 antibody titer after Covishield vaccination among Health Care workers (HCW) and their associated factors.

A prospective single cohort study of health care workers was done in a tertiary care-teaching institute of central Kerala from January 2021 to October 2021. HCWs who have given pre-vaccination serum sample for SARS-CoV-2 antibody estimation and negative for SARS-CoV-2 antibody were included. They were followed up and their blood samples to check for antibody levels were taken 28 days after first dose, 2 weeks after second dose, and 3 and 6 months after first dose of ChAdOx1 nCoV-19 vaccine. Samples taken from 102 HCW were sent for SARS-CoV-2 IgG antibody testing. Quantitative results were reported as signal to cut-off (S/C) value.

Mean age of the study participants was 39.3 (age range:19-71) yrs. and 71.6% were females. Antibody levels of participants at 3rd month ranged from 0.28 S/C to 21.2 S/C with a mean of 8.01. Only 34 (33.3%) HCW had IgG antibody levels >9.5 S/C. Mean antibody level further declined to 6.09 S/C at 6th month. Only 19 (28.4%) had antibody levels more than 9.5 S/C at 6th month. HCW with aged less than 50 years and those who had COVID disease during the study period had a significantly higher level of IgG antibody titres.

The study found that after vaccination with Covishield vaccine IgG levels peaked at 14 days following second dose of vaccine, then getting decreased in the third month and further in sixth month confirming the need for a booster dose. COVID antibody levels were significantly higher in COVID infected HCW and in young age participants.