Reactogenicity and persistence of IgG antibodies against SARS-CoV-2 among recipients of ChAdOx1 nCoV-19 vaccine: A single center experience from Sri Lanka

Introduction

Vaccination against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was considered the most plausible approach to end the coronavirus disease 2019 (COVID-19) pandemic. Thus, many vaccines were developed within a short duration and rapidly deployed globally. Mitigation of the COVID-19 pandemic was observed after commencing the mass vaccination programs, especially in regions with high vaccination coverage.[1] The ChAdOx1 nCoV-19 was the first approved vaccine in Sri Lanka and the vaccination program commenced on January 29, 2021, with the immunization of healthcare workers in the country.

The acceptability of a vaccine is mainly dependent on its safety profile. The safety of the ChAdOx1 nCoV-19 vaccine was demonstrated in Phase 1–3 clinical trials.[2] Nonetheless, there were safety concerns after commencing the vaccine rollout programs in many countries.[3] Although booster dosing against COVID-19 is recommended, vaccine hesitancy and refusal are observed among people due to the fear of adverse reactions.[4,5] Therefore, data on reactogenicity to COVID vaccines among different populations are essential in alleviating vaccine hesitancy.

The post-vaccination immune response is dynamic and has yet to be elucidated fully. Although humoral immunity constitutes only a part of the immune response to the vaccine, it is far easier to detect due to its widespread use and standardization.[6] Antibodies prevent systemic dissemination of SARS-CoV-2 and protect against mortality.[7] Hence, accurate measurement of the antibody response following vaccination plays an important role in determining the efficacy of the vaccine. Many factors affect the antibody response depending on the individual and the type of vaccine.[8]

A clear understanding of real-world data on immunogenicity is crucial to plan future vaccine strategies in the most cost-effective way, especially in resource-limited settings. Dual comparative investigations of reactogenicity and immunogenicity are essential to ascertain the safety of the vaccines against SARS-CoV-2, the potential need for booster doses in different populations and to decide the interval between periodical boosters. In the present work, we assessed the reactogenicity, humoral immune response, persistence of IgG response and factors associated with antibody waning among a cohort of ChAdOx1 nCoV-19 recipients from Sri Lanka.

Methods

Blood sampling

Blood samples (3 mL) were drawn at three-time points. Sampling was done before the first dose of the vaccine (baseline) to assess the pre-vaccination antibody status of the participants. The other two samples were collected just before the second dose and the booster dose (3 months and nine months after the first dose of the vaccine, respectively) [Figure 1]. Blood sampling was done under strict aseptic conditions by two qualified phlebotomists, where the skin was disinfected with a 70% alcohol swab for 30 s and allowed to dry completely before the collection of samples. Serum was separated according to the standard operating procedures, aliquoted, and stored at −70°C at the research laboratory of KDU until the antibody test was performed.

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Figure 1

Flowchart of participant selection and serology sample collection

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Results

Demographic characteristics

A total of 494 staff members consented to participate in this study and for the successive blood sampling at 3 months after the first dose and 6 months after the second dose of vaccination, 382 and 221 responded, respectively. Hundred and ninety-six (39.7%) study participants provided blood samples at all 3-time points [Figure 1].

Majority of the study population were below 40 years (339/494, 68.6%) with a median age of 33 years (IQR = 28–43). More than half the participants were males (265/494, 53.6%). Comorbidities were infrequent in this study group (94/494, 19%), with hypertension (33/494, 6.7%) and diabetes mellitus (32/494, 6.5%) being the most reported pre-existing medical conditions. Less than 1% of the study participants were diagnosed with COVID-19, while 57 (11.5%) were quarantined due to high-risk exposures before the vaccination. The sociodemographic characteristics of the participants who provided blood samples at all 3-time points (subgroup) were comparable to the whole group [Table 1].

Table 1 Socio-demographic and health characteristics of the participants

Reactogenicity profile

Of the 494 participants, post-vaccination surveillance was completed by 419 (84.8%) and 410 (83.0%) after the first and second doses of the vaccine, respectively. Following the first dose, 329 (78.5%) participants reported experiencing at least one vaccine-related symptom. In contrast, post-vaccination symptoms were less prevalent (104/410, 25.4%) after the second dose. Systemic reactions were more frequent among the first dose recipients (317/419, 75.9%), whereas local symptoms were predominant (74/410, 18.0%) after the second dose. Fever was the commonest post-vaccination symptom among first dose recipients (241/419, 58.7%), followed by pain/tenderness at the injection site (233/419, 55.6%) and body/muscle aches (208/419, 49.6%). Following the second dose, pain/tenderness at the injection site was the most frequent symptom (72/410, 17.6%) [Figure 2].

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Figure 2

Proportions of reported local and systemic reactions after 1^st dose versus 2^nd dose of ChAdOx1 nCoV-19 vaccine

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The total score of vaccine-related symptoms was calculated and the mean total score was significantly higher following the first dose compared to the second dose (2.75 ± 1.89 vs. 0.36 ± 0.71, respectively, P < 0.001). Post-vaccination symptoms were more frequent among younger participants (<40 years) than in recipients above 40 years. The mean total score of symptoms observed across age groups was statistically significant after the first dose (P < 0.01) but not after the second dose. Females had a high mean total score than men following both doses of vaccination. Nonetheless, the observed difference was statistically not significant [Table 2].

Table 2 Mean total score of vaccine related symptoms stratified by participants’ Characteristics

Humoral immune response

Pre-vaccination antibodies were detected in 30 participants (30/494, 6.1%), indicating previous SARS-CoV-2 infection. The antibody levels were low, with a mean antibody titer of 0.5 U/mL ± 2.6. Two of the three participants, who had a previous diagnosis of COVID-19, tested negative for pre-vaccination antibodies. Noteworthy, most of the pre-vaccination seropositive participants (25/30, 83.3%) were not being quarantined or diagnosed with COVID-19 previously.

Excluding the participants who were lost to follow-up, a positive humoral immune response was observed among 371/382 (97.1%) 3 months after receiving the first dose of the vaccine with a mean antibody titer of 22.5 U/mL ± 14.9. The post-first dose antibody response was more profound among seropositive participants before vaccination than those who were seronegative (32.7 U/mL vs. 21.9 U/mL, P = 0.002). The mean antibody titers were significantly higher among females than in males after the first dose (24.2 U/mL vs. 21.0 U/mL, P = 0.041). At 3 months, post-first dose mean antibody levels were comparable between participants <40 years and more than 40 years. Similarly, no significant difference in the mean antibody titers was observed between participants with comorbidities and without comorbidities.

Six months after the second dose of vaccination (nine months after the first dose), only 221 (44.7%) participants responded, and IgG antibodies were detected in all of them with a higher mean antibody titer (39.3 U/mL ± 14.2) [Figure 3].

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Figure 3

Seroprevalence of IgG antibodies before and after vaccination among recipients of ChAdOx1 nCoV-19 vaccine

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Subgroup analysis of the participants who have completed serological sampling at all 3-time points

Blood samples were drawn from 196 participants at all three-time points and baseline IgG antibodies were positive in 12 (6.1%) of them. Three months after receiving the first dose 97.0% seroconversion rate was observed, where participants with pre-vaccination seropositivity elicited a higher antibody response than individuals who were seronegative at baseline (mean Ab titer 35.3 vs. 23.7 P = 0.015). Six months after the second dose, all 196 participants were seropositive and a rise in the previously measured antibody levels was found in 153/196 (78.1%) of them while declining antibody titers were observed in 43/196 (21.9%). One participant had SARS-CoV-2 infection after the first dose of vaccination. None of the participants reported breakthrough infection following the second dose until the time of the third blood sample collection.

Antibody waning at 6 months showed a significant association with pre-vaccination seropositivity (P = 0.015) and female gender (P = 0.022). Antibody decline was frequent among participants older than 60 years (n = 7/23, 30.4%), while the age category 40–59 years included the least proportion of participants with declining titers (n = 5/47, 10.6%). However, the observed difference in antibody waning across age groups was statistically not significant. Moreover, comorbid status of the recipient had no significant association with declining IgG levels [Table 3].

Table 3 Association between participants’ characteristics and declining IgG antibody titers 6 months after the second dose of vaccination (nine months after the first dose) (n=196)

Discussion

This study sought the reactogenicity and the longevity of the humoral immune response in a cohort dually vaccinated with the ChAdOx1 nCoV-19 vaccine. We found high seroconversion rates with the persistence of IgG antibodies for up to 9 months in the studied population. Pre-vaccination seropositivity and female gender were associated with higher post-first dose IgG levels, as was a faster decay of IgG levels. A significant reduction in reactogenicity was observed following the second dose of vaccination.

In the present study, post-vaccination reactions were reported to be much lower than anticipated in clinical trials.[10] Fever was the predominant systemic reaction to the first dose of vaccination in our population compared to fatigue, muscle aches, and headache reported in numerous studies.[11-13] Notably, two regional studies also reported fever as the commonest systemic symptom following the first dose of vaccination.[14,15] In accordance with previous studies on the ChAdOx1 nCoV-19 vaccine, the reactogenicity to the second dose was significantly less among our population compared to the first dose.[10,14,15] Interestingly, this lower reactogenicity with each additional dose was not common for all vaccines, where the recipients of mRNA vaccines showed more frequent and severe adverse reactions following the second dose.[16] Female gender, young age and prior SARS-CoV-2 infection were associated with high vaccine reactogenicity in the previous studies.[11,12,17] We observed similar high reactogenicity among young adults (<40 years) following the first dose but not after the second dose. No association between participants’ degree of reactogenicity and gender or baseline seropositivity could be identified. Neither thrombotic events linked to the vaccine nor life-threatening adverse reactions were observed in our population.

Despite our study population being potentially more at risk of being exposed to COVID-19 patients, the pre-vaccination seropositivity was lower (6.1%) than that of the general population (24.46%).[18] Participants with pre-vaccination seropositivity demonstrated a more robust post-first dose immune response in terms of the magnitude of antibody levels than the seronegative group. The pre-vaccination seropositivity indicates the previous SARS-CoV-2 infections despite almost all being asymptomatic and undiagnosed in our population. The natural infection stimulates B and T cells, whereas B cells get stimulated following vaccination. Therefore, the observed profound antibody response following the first dose among pre-vaccination seropositive participants is attributed to the vaccine’s booster effect and has reported in the literature.[19,20]

When comparing the seroconversion rates and antibody tires following the first and second doses, our findings were in accordance with the original phase 2/3 trials. The clinical trials observed the development of IgG antibodies within 2 weeks after vaccination and a further rise in titers with a seroconversion rate of 99% at 14 days following the second dose.[21] However, the post-vaccination seropositivity rates in the real-world data were inconsistent across the various geographical areas. A study from Saudi Arabia reported a lower seroconversion rate (79.6%) after the first dose of the ChAdOx1 nCoV-19 vaccine but 98.3% seropositivity following the second dose.[22] The post-vaccination seropositivity at 28 days after the first dose was 81.9% among healthcare workers in Eastern India.[23] In the same subcontinent, the seroconversion rate was 69.7% among healthcare workers in a cardiac center following the second dose of vaccination.[24] Our study observed 97.1% and 100% seropositivity following the first and second doses. Compared to the 4–6 weeks dosing interval in the Indian studies, a 12-week gap was maintained between the two doses in Sri Lanka to ensure maximum deployment of the first doses to many people in the country. The robust seroconversion rates in our study may reflect the effectiveness of the adapted vaccination strategy. Better immunogenicity and higher vaccine efficacy had been reported in the previous studies with a longer interval between the two doses.[25]

On evaluating the durability and magnitude of IgG response among 196 participants who provided blood samples at all-time points, a rise in the previously measured antibody titers were observed in the majority 6 months after the second dose (nine months after the first dose). None of them reported SARS-CoV-2 infection between the second and third blood sample collection. Nonetheless, the possibility of asymptomatic infection cannot be ruled out and may have influenced the antibody titers at 6 months. Although the kinetics of antibody response to vaccines against SARS-CoV-2 infection is not fully understood, waning has been observed after immunization.[26] The durability of the post-vaccination humoral immunity and the degree of antibody decline reported in the previous studies were incongruous. In addition to the variations in vaccine regimes, geographical and racial differences and exposure to SARS-CoV-2 may influence recipients’ post-vaccination humoral immune response. A 6-month follow-up study of Indian healthcare workers revealed a two-fold decrease in antibody titers among ChAdOx1 nCoV-19 recipients following the second dose of the vaccination, where the decline in IgG levels was observed from the 3^rd month onwards.[23] A study among 30 healthcare workers in India showed a consistent rise in the antibody titers up to 3 weeks after the first dose and a steady fall from the 1^st week after the second dose. The reported trend was common for both previously SARS-CoV-2 infected and non-infected individuals.[19] Thus, persistent higher antibody titers observed among more than two-thirds of our study population is exceptional.

The protection imparted by the IgG antibodies against SARS-CoV-2 infection and the required protection threshold has not been established yet.[27] Nonetheless, the significant correlation between neutralizing and anti-S IgG antibodies shown in the previous studies suggests that these antibodies are effective in neutralizing the virus.[28,29] IgG antibodies can be detected easily in most laboratories. Hence, a selective booster vaccination strategy than a mass vaccination approach based on IgG levels is presumably more cost-effective, especially for developing countries. Such a targeted immunization strategy to maximize the limited resources imparts the need for further studies on the usefulness of IgG antibody levels in booster dose decisions.

Despite profound immune response to the first vaccination dose, the significant antibody waning among females and pre-vaccination seropositive participants following the second dose is striking. In contrast to our finding, immunization after SARS-CoV-2 infection had shown a more durable humoral immune response as compared to two-dose vaccination in a SARS-CoV-2-naïve Individuals.[30] Nonetheless, confirming the findings of many studies, the antibody decay showed no statistically significant difference across age or according to comorbid status.[23] As old age and comorbidity are predictors of severe COVID-19, the observed sustained humoral immunity following vaccination in these groups suggest prolonged protection against severe disease.[31,32]

To the best of our knowledge, the present study is the first to compare the reactogenicity and immunogenicity of the ChAdOx1 nCoV-19 vaccine in Sri Lanka. However, given our limited resources, blood samples were drawn only at 3-time points for the evaluation of humoral immune response and neutralizing titers were not estimated. This is the main limitation of our study, where serial blood samples would have given a better understanding of the antibody response. We only studied the reactogenicity up to 1 week following each dose of the vaccine and the lack of data on long-term adverse effects of the vaccine is another limitation. The study included only a cohort of hospital staff, which limits the generalizability of the results. Nevertheless, this study provides insight into adapting comprehensive and effective vaccination strategies, especially in a resource-limited setting.

Conclusions

Although post-vaccination symptoms were common after the first dose, a significant reduction in the reactogenicity was observed following the second dose. None of the participants had vaccine-related thrombotic events or life-threatening adverse reactions. Post-first dose seroconversion rates were high, and the participants remained seropositive even nine months after the first dose. Therefore, the ChAdOx1 nCoV-19 vaccine is safe and immunogenic in the studied population. The observed sustained humoral response following vaccination with waning among a small proportion of participants suggests the possibility of selective booster immunization based on IgG levels than mass vaccination of populations against SARS-CoV-2. However, the significant waning of antibodies following the second dose among pre-vaccination seropositive participants and females needs broader population studies for a better understanding of the dynamics of immune response to the ChAdOx1 nCoV-19 vaccine.

Author’s Declaration Statements

Author Contributions

Conceptualization: DG, Design of research study: DG, DN, WW, NSF, USS, Data acquisition: DG, USS, TDW, DN, TNS, PK UK, Data analysis: DG, DD, WW, USS, NSF, PK, TNS, UK, Interpretation of results: DG, DD, DL, NSF, PK, TNS, UK, Writing the manuscript: DG, DD, DL, WW, ADS, NSF.

Funding acquisition: DG, All authors contributed and approved the final draft of the manuscript.