SARS-CoV-2 infection despite high levels of vaccine-induced anti-RBD antibodies: a study on 1110 healthcare professionals from a northern Italian university hospital

Abstract

To the Editor, SARS-CoV-2 vaccination campaigns are at an advanced stage in many countries, yet little concerns raised about some cases of post-vaccination infection (1). Thus, real world data are needed to guide health policy makers especially now that the implementation of a third dose administration protocol is being discussed. Thanks to a longitudinal study (Covidiagnostix), funded by the Italian Ministry of Health, we investigated the antibody (Ab) response, over a 6 months period, in 1110 healthcare professionals (HCPs) injected with both doses of the BNT162b2 vaccine (January-February, 2021) at the San Raffaele Hospital (HSR) in Milan, Italy. HCPs previously infected by SARS-CoV-2 were identified by testing their sera, few minutes before the 1st vaccination dose (T0), for the presence of Abs against the viral nucleocapsid-protein (N) (Roche Anti-SARS-CoV-2, ECLIA). HCPs were further tested after 21 (T1) (right before the 2nd vaccination dose), 42 (T2) and 180 (T3) days for the presence of Abs against the Receptor Binding Domain (RBD) of the viral spike-protein (S) (Roche Anti-SARS-CoV-2-S, ECLIA). As part of a follow up program, HCPs were also sporadically subjected to RT-PCR amplification tests of nasopharyngeal swabs as well as to serological tests at time points different from those of the Covidiagnostix. At T0 90 HCPs (8.2%) were anti-N seropositive and showed the previously observed exceptional anti-RBD titer increase at T1 upon receiving the 1st dose (2). The remaining 1020 seronegative HCPs showed the production of anti-RBD Abs, upon receiving the 1st dose (T1), which was boosted by the 2nd dose administration (T2) (2) and followed by a decrease of approximately 70% at T3 in the majority of the HCPs (n=929, 91.1%). The remaining group (n=91, 8.9%) showing T3 minus T2 anti-RBD titers ≥0 was tested (T3) for the presence of anti-N Abs. Ten of them resulted positive, indicating post-vaccination infections. As a control group, 410 HCPs showing T3 minus T2 titers ≤0 were also tested for the presence of anti-N; all of them were negative and none showed positive RT-PCR swab test. Two more post-vaccination infected HCPs, showing T3 minus T2 Ab titers <0 (Table 1 , subjects 11, 12), were identified through post-vaccination RT-PCR swab tests. Their infection was confirmed by the detection of anti-N serum Abs at T3. Table 1 Demographic characteristics, serological results and COVID-19 related information of the 12 HCPs post-vaccination infected by SARS-CoV-2. Table 1 Subject Sex Age (years) Anti-RBD (BAU/mL) PCR cyclesa Type of variant 2nd dose to infection interval (days)b Symptoms Close contactsf Time length of negativizationc (days) HCP position T1 T2 T3 RdRp gene EGene 1 M 76 2.93 2122 >2500 22.9 20.9 B.1.1.7 64 Asymptomatic Yes 21 Institutional Review Board 2 F 42 142 >2500 >2500 34.1 N/A N/A 59 Asymptomatic Yes 13 Psychologist 3 M 54 <0.4 196 897 25.6 24.2 N/A 69 Partial anosmia/ageusia Yes 13 Nurse (Pediatric) 4 M 39 1019 1866 >2500 N/A N/A N/A N/Ad Asymptomatic No ? Administrative 5 F 57 <0.4 2047 >2500 N/A N/A N/A N/Ad Asymptomatic No ? Administrative 6 F 55 208 >2500 >2500 N/A N/A N/A <0e Asymptomatic No ? Nurse (Infectious Diseases) 7 F 49 77.5 >2500 >2500 22.3 22.1 B.1.1.7 67 Partial anosmia/ageusia, cold, generalized pain Yes 13 Nurse (Psychiatry) 8 M 53 0.79 339 >2500 223.1 23.5 B.1.1.7 84 Asymptomatic No 16 Technician (Echography) 9 F 42 18.5 1131 >2500 30.8 31.8 N/A 42 Asymptomatic Yes 14 Nurse (Psychiatry) 10 F 55 76.3 2046 >2500 30.1 30.3 N/A 99 Partial anosmia/ageusia, cold Yes 22 Technician (Pathological Anatomy) 11 F 42 50.1 >2500 2495 21.4 20.8 N/A 14 Partial anosmia/ageusia, cold Yes 14 Nurse (General medicine) 12 F 56 5.7 1066 714 28.1 27.8 N/A 7 Asymptomatic Yes 17 Nurse (Cardiology Department) a Values refers to the first positive swab test. Values were considered: positive (between 14 and 34) slightly positive (between 34 and 40), negative (>40). b Intervals are calculated from the day of the 2nd dose to the day of the first positive RT-PCR test. c Time length of negativization was calculated from the day of the first positive RT-PCR test to the day of the first negative RT-PCR test. d COVID-19 was asymptomatic and the HCPs found out about the infection only through the serological test at T3. e Positivity was discovered by an occasional anti-N test performed at T1. f ”Close contacts” refers to the presence of a SARS-CoV-2 positive unvaccinated household at the time of infection. Eight post-vaccination infected HCPs were females aged 49.8±6.8 and 4 were males aged 55.5±15.3 (Table 1). One subject was infected between the 1st and the 2nd vaccine dose, nine were infected between 7 and 99 days after the 2nd dose whereas 2 HCPs were oblivious of having being infected (Table 1). All the subjects were asymptomatic except 4 reporting partial anosmia and ageusia accompanied, in 3 cases, by a mild cold and, in one of the latter cases by a generalized pain (Table 1). The possibility of an in-hospital outbreak was ruled out since the 12 HCPs perform, within the hospital, different tasks (Table 1) with the exception of 2 nurses from the Psychiatric Department which, however, were infected one month apart. Notably, 8 out of 12 post-vaccination infected HCPs reported the presence of a SARS-CoV-2 positive family member (not vaccinated) as potential source of infection (Table 1). Reduced vaccination efficiency has been observed in older individuals (>60 years old) (2), yet, the 12 HCPs were between 39 and 57 years old, except one that was 76 years old. Interestingly, 7 subjects had anti-RBD titers at T2 above 2000 BAU/mL, 3 were between 1000 and 2000 BAU/mL and only 2 had titers <400 BAU/mL (Table 1). An anti-RBD titer threshold of approximately 1300 BAU/mL was associated with neutralizing activity as previously described by Ferrari et. al (5). Although the latter is not the only correlate for vaccine efficacy, with memory B and T cells possibly playing a key role in protection, we would have expected a better consistency between high anti-RBD Ab serum levels and protection from infection. These data further highlights the difficulty to find a reliable and unique correlate of protection by assessing the serum neutralizing antibody titers only. It must be noted that two HCPs (subject 3 and 5) did not respond to the first vaccine dose and showed T1 anti-RBD titers <0.4 U/mL (Table 1). In conclusion, 6 months after vaccination of 1110 HCPs 12 of them were infected despite receiving proper BNT162b2 administration protocol (except one infected between the two doses). However, because a fraction of the HCPs was not subjected to anti-N serological test, the number of infections might be underestimated. Post-vaccination infections, distributed throughout the whole observation period, were often associated to the presence of unvaccinated SARS-CoV-2 infected households. Importantly, no in-hospital (or related public areas) secondary cases were observed amongst colleagues (>95% of the HSR employees were vaccinated). Our study showed that, in the observed cohort of HCPs, no severe clinical manifestations of COVID-19 occurred. We might speculate that the latter is the consequence of the efficacy of the BNT162b2 vaccine, yet, infection, as well as symptomatology, were not related to a low anti-RBD Ab response. Under the light of these data, we think that implementation/modification of current vaccine protocols should focus on further studies evaluating clinical outcomes in post-vaccine infected subjects, their anti-RBD Ab titer and, importantly, the possible key role of memory immunity in the protection from severe COVID-19. Transparency declaration The authors declare that they have no conflicts of interest. Author contributions DF designed the study, performed the data analysis and interpretation and wrote the article; NC, NM and ML wrote the article and supervised the study. Research Funding This project was supported by Ministry of Health of Italy, “Bando Ricerca COVID-19”; project number: COVID-2020-12371619; project title: COVIDIAGNOSTIX - Health Technology Assessment in Covid serological diagnostics. Statement on research ethics The COVIDIAGNOSTIX study was approved by the San Raffaele Hospital Institutional Ethical Review Board (CE:199/INT/2020).ht. Uncited reference 3., 4..