Study assesses antibody titres and protection against SARS-CoV-2 infection

The study involved 8,758 HCWs with a median age of 40 years (interquartile range [IQR] 32-50), and the majority (80.4%) of the cohort was female. Over half of them (54.9%) had received either one dose (n = 2,244) or two doses (n = 2,567) of vaccine between January and April 15, 2021 of the Oxford–AstraZeneca ChAdOx1 nCoV-19 (AZD1222) or Pfizer-BioNTech COVID-19 mRNA (BNT162b2) vaccine. Neutralising antibody (NAb) titres for vaccinated HCWs were obtained three weeks after administration of the first or second vaccine dose.

An average of 9.65% (range 7.2%-12.1%) of the HCWs who had no NAb became infected after a median follow-up of 275 (IQR 265-281) days, as did 2.2% (95% confidence interval [CI] 0.4%-4%) of those with a NAb titre well below 64. In contrast, only 0.6% (95% CI 0% -1.5%) of those with NAb titres of 64 to 128 became infected, whereas none of those with NAb titres of 256 and above (P < 0.01, Chi2 test) were infected. 

Meanwhile, analysis of ELISA total antibody concentrations indicated that 12.1% (95% CI 11.5%-12.8%) of HCWs with a negative ELISA or an ELISA concentration below 13 BAU/ml became infected with SARS-CoV-2 between July 2020 and April 2021, as did 10.6% (95% CI 6.5%-16.1%) of HCWs that had an ELISA concentration between 13 and 141 BAU/ml. On the contrary, only 1.3% (95% CI 0.03%-7.2%) of those with an ELISA concentration between 141 and 1,700 BAU/ml became infected, while none of those with an ELISA concentration of 1,700 BAU/ml and above (P < 0.01, Chi2 test) were infected. 

“Analysis of all the data indicated that a NAb titre well below 64 provided 76.8% protection against SARS-CoV-2, a titre of 64 to 128 gave 94% protection and a NAb titre of 256 or more provided full (100%) protection,” wrote researchers led by Chloé Dimeglio, Toulouse University Hospital, Toulouse, France. “In the same way, an ELISA concentration between 13 and 141 BAU/ml provided only 12.4% protection against SARS-CoV-2, a concentration between 141 and 1,700 BAU/ml provided 89.3% protection and a concentration of 1,700 BAU/ml and above provided full protection.”

“Our study did not assess cell-mediated immunity and all the subjects were HCWs. However, the data suggest that monitoring the NAb response but also total antibody concentrations, logistically more feasible, can be used to optimise vaccination strategies by estimating the duration and degree of protection provided by vaccines,” the researchers noted, adding that “the thresholds of protection found in [the] study should be compared to those obtained in further studies on other populations.”