A study on seroconversion following first & second doses of ChAdOx1 nCoV-19 vaccine in central Kerala : Indian Journal of Medical Research

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A study on seroconversion following first & second doses of ChAdOx1 nCoV-19 vaccine in central Kerala

Varghese, Sangeetha Merrin1,; Mateethra, George Chandy2; George, Geomcy3; Chandran, Vishnu S.4; John, Grace Mary; Varghese, Levin Thamban; Mammen, Nithin K.; Vinayak, V.

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Indian Journal of Medical Research 155(5&6):p 499-504, May–Jun 2022. | DOI: 10.4103/ijmr.ijmr_1917_21
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The COVID-19 pandemic has been a serious threat to the mankind. In this scenario, it is important to know the efficacy of the vaccines used to immunize our population. The first vaccine to be launched in India on January 16, 2021 was ChAdOx1-nCoV vaccine . It is a modified chimpanzee adenoviral - vector vaccine, claiming efficacy of 76 per cent against symptomatic infection in recipients who took the vaccine at least 22 days apart1. The vaccine causes the expression of SARS-CoV-2 spike protein gene, which in turn induces host cells to produce the protein of S-antigen. This allows the body to generate an immune response and retain the information in the memory immune cells. Vaccination against COVID-19 induces spike-protein-binding IgG antibody levels and a robust correlation has been observed between antibody titre and efficacy2. However, it is important to note that correlation does not imply causation, and factors other than antibody responses may also play an important role in impacting efficacy.

Although some vaccines offer life-long protection, for many diseases, but immunity wanes over time3. This study compared the antibody titres following both the first and second doses to have a closer look at the waning effect of immunity in our setting. In the case of COVID-19, newer more transmissible variants are also a threat, compounded with the worldwide deficit in the vaccine supply chain. In this scenario, it would be worthwhile to think about a flexible immunization schedule allowing to mix and match vaccines, which could possibly trigger a more potent immune response4 and also have an additional benefit in providing both prime and booster doses of vaccine against SARS- CoV-2 within the recommended dosage interval.

Understanding the dynamics of post-vaccine antibodies after both the first and second doses and how these differ between individuals by age, gender, co-morbidities and previous COVID-19 infection become important. To understand the immunogenicity of the ChAdOx1 nCoV-19 vaccine, a cross-sectional study was conducted on healthcare workers of a tertiary care health centre in South India to assess the seroconversion (antibody responses) after vaccination.

Material & Methods

The study was conducted at Believers Church Medical College, Thiruvalla, Kerala, India, a tertiary care centre, during February-April 2021 after obtaining clearance from the Institutional Ethics Committee (IEC 2021/04/203). Informed consent was obtained from all participants, which also included permission for blood collection at pre-determined intervals.

The study participants were 200 healthcare workers (HCWs) - doctors, nurses and supporting staff, who were enrolled for vaccination during the first three days of the first phase (January 16-February 28) of the COVID-19 vaccination drive. These HCWs were randomly selected by the district health officials through a computer-generated random sampling, from the staff list in October 2021.

Ramasamy et al5 reported that seroconversion after the first dose of the ChAdOx1 nCoV-19 vaccine was 91 per cent. Hence, to estimate the true population proportion with precision-4 and 95 per cent confidence level, the required sample size was 180.

A qualitative immune-chromatographic card test (Sensit Rapid COVID-19 IgG/IgM, UBIO Biotech, Cochin) was done to detect COVID-19 IgG antibodies in all vaccine beneficiaries before vaccination to serve as a benchmark for the comparison of results after vaccination. Those who contracted COVID-19 after the first dose of the vaccine, were excluded from the study. Staff with previous documented COVID-19 infection either by reverse-transcription polymerase chain reaction or by antigen test or positive on IgG card test were also included in the study (if they were in the list generated by random selection) to evaluate any difference in antibody response to vaccination among those with previous COVID-19 infection.

Vaccine beneficiaries were contacted after 28 days of receiving their first dose and between 28-35 days after their second dose of ChAdOx1 nCoV-19 vaccine for blood collection. Seroconversion was measured at 28 days as is on the package insert of ChAdOx1 nCoV-19 vaccine56. Both doses of vaccine were taken within a gap of 28-42 days. Antibody test was performed using the VITROS® Anti-SARS-CoV-2 IgG (Ortho-Clinical Diagnostics, Rochester, NY, USA) a test for the detection of serum IgG antibodies to the immune-dominant S1 spike of the virus protein of SARS-CoV-2, having a sensitivity of >90 per cent and specificity of 100 per cent7. The VITROS anti-SARS-CoV-2 IgG test was approved for use under the Food and Drug Administration’s Emergency Use Authorization. Seroconversion was defined as having detectable anti-SARS-CoV-2 IgG antibodies ≥1 on the test. Interpretation of results was as follows: a signal-to-cut-off ratio (S/Co) ≥1 was indicative of a positive result indicating seroconversion. A value <1 was negative for seroconversion. Antibody titre 12 or greater was considered as a high titre while measuring for anti-SARS-CoV-2 antibodies89.

Data collection was done in the vaccination area itself, when the beneficiaries came for their immunization by four clinical pharmacy specialists. They were recruited for data collection after a training session and a quality check was done by the principal investigator for five per cent of the samples. Data included socio-demographic details, antibody titre value after the first and second doses of vaccine, details about comorbidities, the habit of exercising (more than 30 min for at least three days in a week), presence of any perceived stress or decreased sleep (less than six hours of sleep for more than three days in a week).

Statistical analysis was performed using the SPSS software v.21 ( IBM Corp. Released 2012. IBM SPSS Statistics for Windows, Version 21.0. Armonk, NY, USA: IBM Corp.). The categorical variables were expressed as percentages. Chi-square was used to test for associations. Mean antibody titres across different groups were compared using a t test. Logistic regression analysis was also done. Box and Whisker plot was used to display the variation in antibody titres across different groups.

Results & Discussion

The study was conducted on 200 vaccine beneficiaries 28 days after the first dose and between 28 and 42 days after receiving the second dose of the vaccine. Majority of the vaccine recipients belonged to the age group of 30-50 years. Their mean age was 39.94±11.24 yr (minimum-24 yr, maximum-75 yr). One hundred and twenty two (61%) of the vaccine recipients were females. Previous documented COVID-19 infection was present in 10.5 per cent (n= 21/200) of the vaccine beneficiaries. Twenty one were positive on the immunochromatographic card test for COVID -19 IgG antibodies.. All these people had a previous history of documented COVID-19 infection. A total of 19 per cent had either hypertension (HTN) or diabetes and 27 per cent were on treatment for chronic diseases.

Seroconversion after the second dose of the vaccine was seen in 99.5 per cent (199/200) of the beneficiaries as compared to only 91.5 per cent (183/200) after the first dose of the vaccine. After excluding those with previous COVID-19 infection (n=21), the second dose attained seroconversion in 99.4 per cent (178/179) and the first dose attained seroconversion in 90.5 per cent (162/179) of the vaccine beneficiaries. The mean titre after the second dose was significantly (P<0.001) higher when compared to the first dose (12.68±4.17 vs. 9.83±6.3). There was an antibody titre ≥12 in 54 per cent (n=108) of the vaccine recipients. Antibody titres did not have any significant association with age, gender, co-morbidities (HTN and diabetes) or factors such as exercise, sleep or stress (Table I). The single most important factor that was associated with high antibody titre was prior infection with SARS CoV-2 (P=0.026).

Table I:
Factors influencing antibody titres after the second dose of COVID-19 vaccination

Participants with prior COVID-19 infection were detected to have higher immunogenicity in comparison to SARS-CoV-2 naïve participants. Antibody titre after the first dose in those without previous COVID-19 (n=179) was 8.69±7.43 and with previous COVID-19 infection (n=21) was 20.5±3.7. Antibody titre after the second dose in those without previous COVID-19 was 12.8±3.7 and with previous COVID-19 was 17.3±2.8. Those with previous COVID-19 infection and only one vaccine shot had a titre [mean-17.81±5.94, median-20.5 (IQR-3.7)] significantly higher than that found in SARS CoV-2 naïve individuals even after the second dose [mean-12.29±4.00, median-12.8 (IQR-3.7), P=0.001]. However, among those with previous COVID-19 infection, there was a fall in titre after the second dose [mean-16.06±4.12 and median-17.3 (IQR-2.8)] of the vaccine when compared to the titre after the first dose [mean-17.81±5.94, median-20.5 (IQR-3.7)].

A decrease in titre following the second dose of vaccination was seen in 28 per cent (n=56) of the vaccine beneficiaries. The mean fall in titre was 2.25±1.40. Previous COVID-19 infection and gender being a male were significantly associated with a decrease in titre following the second dose of vaccination (Table II). Furthermore, the older age group did not have a fall in titre after the second dose when compared to the younger age group. Among those with previous COVID-19 infection, nearly three-fourths of them had a fall in titre following the second dose (Table II). Despite a reduction in titre after the second dose, it was still significantly (P<0.001) higher when compared to SARS-CoV-2 naïve individuals (16.06±4.02 vs. 12.29±4.00). Fall in antibody titre did not have any significant association with co-morbidities such as HTN and diabetes (Table II).

Table II:
Factors associated with decreased titre following vaccination

Seroconversion following the second dose of the vaccine was seen in 99.5 per cent of the vaccine recipients. In a study by Eyre et al10, all HCWs assessed 14 days after the second dose were seroconverted. Seroconversion following ChAdOx1 nCoV-19 vaccine was reported to be 91 per cent11, Sputnik V - 88.9 per cent12 and CoronaVac (Sinovac Life Sciences, Beijing, China) 83.3 per cent in healthy adults12. The efficacy after the second dose of the ChAdOx1 nCov-19 vaccine was 70 per cent (55%-81%)12 and the seroconversion rate was claimed to be 98 per cent on the product package insert7. Therefore, there was an incomplete relationship or absence of any linear correlation between seroconversion and protection from COVID-19, as seroconversion rate post-vaccination exceeded the proportional reduction in the incidence of COVID-19. Thus, the best correlate of protection may be a combined measure of cellular and humoral immunity, clinical outcomes, and antibody responses, which should contribute to prioritization decisions. The main determinant of antibody responses after the second dose was a previous COVID-19 infection. This was also true for studies conducted with mRNA vaccines (i.e. after a single dose of the Pfizer-BioNTech vaccine), people with a prior COVID-19 infection had antibody levels similar to those who had taken two doses and without prior COVID-19 infection13. Khoury et al14 reported a decline in neutralization titre eight months after SARS-CoV-2 infection.

Among those with previous COVID-19 infection, the majority (71.4%) had a fall in titre after the second dose of vaccination. This could be due to the possible synergistic effect of the anti-spike IgG antibodies after natural infection and the vaccine, leading to initial high antibody titres (not solely due to vaccination) and the natural antibodies waning over time.

Despite not having a previous COVID infection, 23 per cent of the vaccine recipients had a fall in titre after the second dose of the vaccine. This may be a matter of concern with the vaccines using adenoviral vectors. One suggested possibility is the formation of anti-adenoviral antibodies after the first dose of the vaccine, which could have nullified the effect of the second dose due to an immune reaction to the vaccine vector itself. This would have led to a reduction in anti-spike antibodies after the second dose of vaccination15. Although older people, males and those with long-term health conditions were commonly low responders16, it was noted that patient characteristics such as age and gender and co-morbid conditions such as diabetes did not have any significant impact on the antibody titres after the second dose of vaccine in our population.

Long-term vaccine efficacy requires the persistence of vaccine antibodies above protective thresholds and/or the maintenance of immune memory cells that could rapidly and effectively reactivate with subsequent microbial exposure. Although some studies support that SARS-CoV-2 antibody levels declined with time, others have shown that immune cells can also provide protection from the virus over time12. Repeated doses of virus-based vaccines tend to be increasingly less effective because of an immune response mounted against the adenovirus. Mixing and matching vaccines could be another viable, more potent option4.

Limitations of the study were the lack of evidence of cell-mediated immunity in establishing vaccine efficacy and enrolling only HCWs, who were not the representative of the larger population. Additional research is needed to understand about fall in antibody titres after the second dose and also the duration of protection provided by humoral immunity.

In conclusion, the study suggests that seroconversion after the two doses of the COVID-19 vaccine was 99.5 per cent. Greater immunogenicity was seen among those with previous COVID-19 infection, with a mean antibody titre higher than that found even after the second dose in SARS-CoV-2 naïve vaccine recipients. A fall in antibody titre following the second dose was observed in 28 per cent of vaccine recipients. Except for the presence of a previous COVID-19 infection, antibody titres following the second dose did not have any association with age, gender or co-morbidities in the vaccine recipients.

Financial support & sponsorship: The study was funded by the Believers Church Medical College, Kerala, India.

Conflicts of Interest: None.


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ChAdOx1 nCoV-19; immunogenicity; Oxford AstraZeneca vaccine; second dose; seroconversion

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