Immunological memory and neutralizing activity to a single dose of COVID-19 vaccine in previously infected individuals.

BACKGROUND: The efficacy of COVID-19 vaccines to generate immunological memory post-vaccination has not previously been studied.
OBJECTIVE: To assess immunological memory in previously SARS-CoV-2 infected individuals after a single dose of mRNA vaccine. PATIENTS AND METHODS: Healthcare workers (n = 280) were enrolled after obtaining written informed consent and grouped under previously infected and no prior exposure (reverse transcription-polymerase chain reaction positive and negative, respectively). Blood was drawn at baseline and post-vaccination (single dose of COVISHIELD) for enumerating neutralizing antibodies by chemiluminescence and memory T- and B-cells by flow cytometry.
RESULTS: Post vaccination, compared with the no prior exposure group, the previously infected group had higher levels of: antibody response (1124.73 ± 869.13 vs 94.23 ± 140.06 AU/ml, p = 0.0001); CD4 memory T-cells: central memory CCR7+CD45RA- (p = 0.0001), effector memory CCR7-/CD45RA- (p = 0.01); total CD8+ T-cells (p = 0.004); CD8+ naïve T-cells CCR7+CD45RA+ (p = 0.01); and memory B-cells CD20+CD27+ (p = 0.0001). DISCUSSION: Single-dose vaccination elicited higher neutralizing antibody response and protective immunity in individuals who had recovered from SARS-CoV-2 infection compared with those with no prior exposure.

Introduction

The ongoing COVID-19 pandemic caused by SARS-CoV-2 has resulted in unprecedented mortality and morbidity globally. In the absence of specific therapeutic drugs to treat COVID-19, vaccines are currently the only option to control SARS-CoV-2 infection (Krammer, 2020). COVID-19 vaccination programs initiated worldwide have reduced disease severity (Rinott et al., 2021). Vaccinating huge populations require evidence-based strategies for successful implementation to control the pandemic. Although individuals who have had COVID-19 have been shown to have a higher antibody response to a single dose of mRNA vaccine than non-exposed individuals (Krammer et al., 2021, Saadat et al., 2021), their efficacy to generate immunological memory and protection against reinfection of SARS-CoV-2 is not yet reported. If a single dose could induce adequate immunological memory in previously infected individuals in addition to a higher neutralizing activity, the second dose could be diverted to vaccinate and protect a larger population. Therefore, our aim was to assess immunological memory in previously infected individuals after a single dose of vector-based vaccine.

Participants and methods

We enrolled healthcare workers (n = 280) who were vaccinated between 16 January and 5 February 2021, at AIG Hospitals, Hyderabad, India (a hospital recognized by the Indian Council of Medical research to test, treat and vaccinate for COVID-19) for assessing immunological memory response. Participants who were reverse transcription-polymerase chain reaction (RT-PCR) positive for SARS-CoV-2 and recovered formed the previously infected group, and participants who were RT-PCR negative formed the no prior exposure group. All participants were given COVISHIELD (AstraZeneca vaccine ChAdOx1/AZD1222 manufactured by Serum Institute of India) in two doses, 28 days apart, as per the (then) guidelines. Side effects after the single dose of vaccine were noted for all the participants. Blood was drawn from all the participants at day 0 (baseline) and 28 (post single-dose). Serum samples were tested for neutralizing antibodies (immunoglobulin G) by chemiluminescence (Shang et al., 2020), and day 28 samples were analysed for neutralizing antibodies and memory cells by flow cytometry (Yang et al., 2020). The Institutional Ethics Committee of AIG Hospitals approved the study, and all participants provided written informed consent. Students t test and Z proportion test were used to test differences between the groups. A P-value of <0.05 was considered significant.

Results

Participant demographics, baseline serostatus and frequency of side effects after the single dose of vaccination are provided in Table 1. Of the 280 individuals enrolled in the study, 131 were RT-PCR positive with mild to moderate disease, and 50 required hospital admission. All 131 (46.78%) were seropositive, and 149 (53.22%) were seronegative prior to the first vaccination dose.

Table 1

Demographics, serostatus and post vaccination symptoms of participants.

	Previously infected	No prior exposure	Significancea
Participants (n)	131	149	–
Male	79 (60.30%)	98 (65.77%)	0.34
Female	52 (39.69%)	51 (34.22%)	0.34
Age (years)
Male	20–58	18–58	–
Female	19–53	18–60	–
Baseline seropositivity (%)			–
Overall	46.78	–	–
Male	44.6	–	–
Female	50.4	–	–
Duration between infection and vaccination (months)	5.0 ± 2.0	–	–
Time to vaccination after recovery (months)	4.0 ± 2.0	–	–
Type of vaccine	COVISHIELD	COVISHIELD	–
Symptoms (n and % of participants)
Fever	38 (29 %)	28 (18.79%)	0.04
Headache	13 (9.92%)	15 (10.06%)	0.96
Body pains	38 (29%)	25 (16.77%)	0.01
Chills	1 (0.76%)	–	–
Back pain	5 (3.81%)	2 (1.34%)	0.18
Cold, cough	–	2 (1.34%)	–
Fatigue	90 (68.70%)	60 (40.26%)	0.0001
Local side effects	13 (9.92%)	15 (10.6%)	0.96

n – number; % – percentage.

Z-test.

Higher antibody response in previously infected individuals with single dose

Baseline neutralizing antibodies were significantly higher in previously infected individuals compared with the no prior exposure group (61.58 ± 46.88 vs 5.03 ± 2.54 AU/ml, p = 0.0001). All the seropositive (100%, n = 131) and 94.6% (n = 141) of seronegative participants developed neutralizing antibodies by day 28 after the first dose of COVISHIELD. The previously infected group mounted greater antibody response (1124.73±869.13 vs 94.23 ± 140.06 AU/ml, p = 0.0001) to a single dose of COVISHIELD vaccine compared with the no prior exposure group (Figure 1 ).

Figure 1

Neutralizing antibody titres and memory cell responses after a single dose of COVISHIELD in previously infected individuals.

Panel A shows antibody titres against S1 and S2 subunits of spike protein of SARS-CoV-2 with single dose of COVISHIELD in individuals previously infected and in individuals with no prior exposure. Antibody titres were significantly higher (P < 0.0001) in previously infected individuals (1124.73 ± 869.13 AU/ml) compared with individuals with no prior exposure (61.58 ± 46.88 AU/ml).

Panel B shows memory T-cell response after a single dose of SARS-CoV-2 vaccine, COVISHIELD (n = 50 individuals with previous infection, 50 no prior exposure). Significantly higher memory CD4 + T-cell response is elicited by a single dose in previously infected individuals than in those with no prior exposure: central memory T-cells CCR7+CD45RA- (p < 0.0001); naïve T-cells CCR7+CD45RA+ (p < 0.0001); effector memory T-cells CCR7-CD45RA+ (p < 0.01). No significant difference between the groups was observed in CD4+ effector T-cells. CD8+ T-cells (p = 0.004), CD8+ naïve T-cells CCR7+CD45RA+ (p = 0.01) were significantly higher in previously infected individuals, while CD8+ central memory T-cells CCR7+CD45RA- (p = 07) and CD8+ effector memory T-cells CCR7-CD45RA- (p = 0.58) showed increasing trend.

Panel C shows memory B-cell response after a single dose of SARS-CoV-2 vaccine, COVISHIELD. Significantly higher memory B-cell response is elicited by a single dose in previously infected individuals than those who with no prior exposure: total B-cells CD45+CD20+ (P = 0.0005), memory B-cells CD20+CD27+ (p = 0.0001) and activated B-cells CD20+HLADR+ (p = 0.0001).

Single-dose vaccine elicited higher memory T- and B-cell responses in previously infected individuals

Memory CD4+ T-cell responses elicited by a single dose of COVISHIELD were significantly higher in the previously infected group compared with the no prior exposure group: CD4+ T-cells (p = 0.001); CCR7+CD45RA- central memory T-cells (p = 0.0001); CCR7+/CD45RA+ naïve T-cells (p = 0.0001); CCR7-/CD45RA- effector memory CD4+ T-cells (p = 0.01). Effector memory CD4+ T-cells CCR7-/CD45RA+ were not significantly different between the groups (p = 0.10) (Figure 1). Total CD8+ T-cells (p = 0.004) and CD8+ naïve T-cells CCR7+CD45RA+ (p = 0.01) were significantly higher in previously infected individuals, while CD8+ central memory T-cells CCR7+CD45RA- (p = 07) and CD8+ effector memory T-cells CCR7-CD45RA- (p = 0.58) showed an increasing trend at day 28. Effector memory CD8+ T-cells CCR7-/CD45RA+ (p = 0.64) were similar in both groups. Significantly higher B-cell responses were seen in previously infected individuals: CD45+CD20+ B-cells (p = 0.0005); CD20+CD27+ memory B-cells (p = 0.0001); CD20+HLADR+ activated B-cells (p = 0.0001) (Figure 1).

Discussion

Our results demonstrate that previously infected individuals mounted higher immune and memory responses to a single dose of vector-based vaccine compared with those with no prior exposure. Earlier studies reported immune memory to SARS-CoV-2 infection (Rodda et al., 2021, Dan et al., 2021) and higher antibody response to a single vaccine dose in previously infected individuals (Rinott et al., 2021). Our study reports higher memory T and B-cell responses in addition to higher antibody response with a single dose of COVISHIELD given at 3–6 months after recovery from COVID-19. These results suggest protective immune memory in previously exposed individuals after a single dose of vaccine. Such individuals could mount memory recall response on a subsequent encounter with antigen as they have developed adaptive immune memory. Thus, individuals who have had COVID-19 and recovered would have adequate protection with a single dose of vaccine. Our results demonstrate evidence to support a single-dose vaccination strategy for previously infected individuals to increase coverage and protect a larger number of populations. All the individuals with no prior exposure to COVID-19 would be required to take the second vaccination dose. In addition, longitudinal follow-up studies are necessary to assess the longevity of protective memory in order to determine the timing of the second dose.

Conflict of interest

None to declare.

Funding source

None.

Ethical approval

The study was approved by the Institutional Ethics Committee of AIG Hospitals, Hyderabad, India.