Indian Journal of Allergy, Asthma and Immunology

: 2022  |  Volume : 36  |  Issue : 1  |  Page : 23--27

Effectiveness of the chAdOx1 nCoV-19 vaccine in preventing COVID-19 infection

Tarana Sarwat1, Devendra Kumar Singh2, Neeraj Pal Singh3, Satendra Pal Singh1, Dalip Kumar Kakru1,  
1 Department of Microbiology, School of Medical Sciences and Research, Sharda University, Greater Noida, Uttar Pradesh, India
2 Department of Respiratory Medicine, School of Medical Sciences and Research, Sharda University, Greater Noida, Uttar Pradesh, India
3 Department of Community Medicine, School of Medical Sciences and Research, Sharda University, Greater Noida, Uttar Pradesh, India

Correspondence Address:
Dr. Tarana Sarwat
Department of Microbiology, School of Medical Sciences and Research, Sharda University, Greater Noida, Uttar Pradesh


AIM: The study aimed to assess the effectiveness of the ChAdOx1 nCoV-19 vaccine in preventing laboratory-confirmed COVID-19. METHODS: It was a test-negative, case‒control study conducted at Sharda Hospital, Greater Noida, India, between March 2021 and May 2021. An equal number of cases and controls were included in the study after taking proper informed consent. The individuals with positive reverse transcriptase-polymerase chain reaction test reports were taken as cases, whereas those with negative reports were included as controls. Data were analyzed and the groups were compared using multivariable logistic regression to calculate the odds ratio (OR), with adjustment for gender and presence or absence of comorbidities. The effectiveness of vaccine was calculated by the formula (1-adjusted OR) ×100%. RESULTS: On analyzing the data from 560 case‒control pairs, the vaccine effectiveness was calculated as 57.46% (95% confidence interval [CI]: 53.85–61.02) and 60.09% (95% CI: 56.32–63.77) for single dose and two doses, respectively. The effectiveness of complete and single-dose vaccination against the moderate-to-severe disease was calculated as 63.79% (95% CI: 58.58–68.77) and 56.19% (95% CI: 51.30–61.0), respectively. CONCLUSION: The ChAdOx1 nCoV-19 vaccine was found to be effective against COVID-19, with protection after two doses being a little more than that after a single dose. It also proved effective in protecting against the severe form of the disease.

How to cite this article:
Sarwat T, Singh DK, Singh NP, Singh SP, Kakru DK. Effectiveness of the chAdOx1 nCoV-19 vaccine in preventing COVID-19 infection.Indian J Allergy Asthma Immunol 2022;36:23-27

How to cite this URL:
Sarwat T, Singh DK, Singh NP, Singh SP, Kakru DK. Effectiveness of the chAdOx1 nCoV-19 vaccine in preventing COVID-19 infection. Indian J Allergy Asthma Immunol [serial online] 2022 [cited 2023 Mar 26 ];36:23-27
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Full Text


Vaccination is one of the effective measures to prevent diseases and also to reduce morbidity and mortality due to the same. So is true for coronavirus disease-19 (COVID-19). India's major vaccination drive began in January 2021 after the Indian drug regulator authorized the emergency use of Covishield (chimpanzee adenovirus) and Covaxin (inactivated virus) in a phased manner beginning with the health care and frontline workers, gradually roping in the elderly (above 60 years of age) and those aged between 45 and 60 years with comorbidities and finally all adults.[1] Covishield, which is manufactured by Serum Institute of India, Pune, in collaboration with Oxford-AstraZeneca, is a recombinant chimpanzee adenovirus vector vaccine which is replication-deficient and encodes for severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) spike protein. It has accounted for approximately 84% of all vaccine doses in India and has been the only vaccine initially available in many areas, including Greater Noida also.[2] A study with pooled data from four randomized trials showed that this vaccine had an overall efficacy of 66.7% (95% confidence interval [CI] 57.4–74.0) in preventing COVID-19 and around 100% in preventing severe illness or hospitalization.[3]

The emerging variants of SARS-CoV-2 have raised concerns pertaining to vaccine effectiveness. Some of the variants include Alpha (B.1.1.7), Beta (B.1.351), Kappa (B.1.617.1), and Delta (B.1.617.2).[4],[5] Our study aimed to determine the effectiveness of the ChAdOx1 nCoV-19 vaccine in preventing laboratory-confirmed COVID-19 during the surge of cases between March 2021 and May 2021 and also to determine its effectiveness in preventing severe disease. At the time of the study, it was the Delta variant (B.1.617.2) which was predominant globally.


Study design and participants

It was a test-negative, case-control study to assess the effectiveness of the ChAdOx1 nCoV-19 vaccine among individuals (symptomatic/asymptomatic) who attended the Sharda Hospital (Greater Noida, Uttar Pradesh) for reverse transcriptase-polymerase chain reaction (RT-PCR) testing (including health-care workers) for suspected SARS-CoV-2 infection between March 2021 and May 2021. A case was defined as the participant who was positive for SARS-CoV-2 infection by RT-PCR test, whereas the control was defined as the participant with negative RT-PCR results for SARS-CoV-2 infection. People with positive or negative SARS-CoV-2 antigen results alone were not included in the study.

The study was approved by the institutional ethics committee of the School of Medical Sciences and Research and Sharda Hospital, Sharda University (reference no. SU/SMS&R/76-A/2021/47). Written/telephonic informed consent was taken from all the participants.

Data collection

Detailed information on vaccination status, vaccine type, presence of symptoms, comorbidities, and severity was taken either telephonically or through a written questionnaire form. Complete vaccination with protective effect was taken to appear after 14 days of the second dose of vaccine, whereas effectiveness after one dose (partial vaccination) was taken to appear after 21 days of the first dose. Unvaccinated individuals were those who had not received even a single dose of vaccine. Moderate-to-severe illness was defined as a requirement for intensive care unit admission for oxygen support or mechanical ventilation or death of the patient.

Statistical analysis

To detect vaccine effectiveness of 60% for complete vaccination with the ChAdOx1 nCoV-19 vaccine against SARS-CoV-2 infection with a precision of 20% and assuming the vaccine coverage to be 50%, the sample size was estimated to be 560 case‒control pairs. The effectiveness of the vaccine for complete vaccination was calculated by comparing fully vaccinated participants with unvaccinated individuals. For this, we excluded all those who had received only a single dose of vaccine. For partial vaccine effectiveness, participants who were administered only a single dose were compared with unvaccinated individuals, and completely vaccinated individuals were excluded. Data were analyzed using IBM, SPSS 22 software, and the adjusted odds ratio (aOR) with 95% CIs was estimated using a multivariate logistic regression model with gender and comorbidities as the confounding factors in addition to the vaccination status. The effectiveness of the vaccine was calculated by the formula (1-aOR) ×100%.[6]


Between March 1, 2021, and May 31, 2021, 22,892 individuals had undergone RT-PCR tests for COVID-19 in our institution. Out of these, 8700 had tested positive, whereas 14,192 were negative. To recruit the calculated number of cases and controls, we contacted the individuals (both positive and negative), and based on appropriate responses and a complete questionnaire, we were able to select 560 cases and control pairs for our study. The demographic characteristics, comorbidities, and vaccination of included participants are shown in [Table 1].{Table 1}

The median age was 36 years and 41 years among the cases and controls, respectively. Among cases, 17% (n = 96) had one or more comorbidities, hypertension, and diabetes of which, were the most common followed by respiratory and chronic cardiac diseases. All the vaccinated participants had received no vaccine other than Covishield.

Most of the cases had a mild disease which required home isolation only (n = 318, 56.80%), whereas 242 cases (43.20%) had moderate-to-severe illness requiring hospitalization. The majority of the fully vaccinated participants developed mild illness (n = 107), whereas only 33 of those developed moderate-to-severe illness with only one death reported among those.

[Table 2] shows the clinical characteristics of COVID-19 disease among the vaccinated and unvaccinated cases.{Table 2}

To calculate the effectiveness of two doses of the ChAdOx1 nCoV-19 vaccine against SARS-CoV-2 infection, 154 cases and 209 controls were included in the analysis giving the vaccine effectiveness of 60.09% (95% CI: 56.32–63.77). The vaccine effectiveness of single-dose against SARS-CoV-2 infection was calculated to be 57.46% (95% CI: 53.85–61.02) where 201 cases were partially vaccinated as compared to 230 partially vaccinated controls as shown in [Table 3]. Out of 155 cases of moderate-to-severe illness, only 33 were fully vaccinated compared to 107 of 204 cases with mild disease. Thus, the effectiveness of complete vaccination against moderate-to-severe illness was 63.79% [95% CI: 58.58–68.77; [Table 3]]. Among 209 cases with moderate-to-severe illness, 87 were vaccinated with a single dose compared to 114 of 211 with mild disease. Hence, the single-dose effectiveness of the vaccine against moderate-to-severe infection was calculated as 56.19% [95% CI: 51.30–61.0; [Table 3]].{Table 3}


The vaccine remains the only effective strategy to mitigate the SARS-CoV-2 pandemic when no other effective antiviral therapy is available. The ChAdOx1 nCoV-19 vaccine is one of the most common vaccines being used in India. In this study, we determined the effectiveness of the ChAdOx1 nCoV-19 vaccine in preventing SARS-CoV-2 infection and also its effectiveness in reducing the severity of the disease.

Our results showed that the vaccine had an association with a reduced risk of COVID-19 infection and was also associated with reduced disease severity. Complete vaccination was 60.09% effective against laboratory-confirmed COVID-19 infection during the surge of cases caused by the Delta variant in India. A test-negative, case‒control study conducted at Faridabad, India, reported 63.1% effectiveness of complete vaccination around the same period.[6] Another study conducted in Vellore, India, reported a 65% effectiveness of the ChAdOx1 nCoV-19 vaccine in reducing the risk of infection.[7] The effectiveness of complete vaccination in preventing infection was reported to be 54% in another test-negative, case‒control study conducted in Puducherry, India.[8] The effectiveness of a single vaccine dose in preventing COVID-19 infection was found to be 57.46% in our study, whereas it was reported to be 46.2% and 49% in studies done at Faridabad and Puducherry, respectively.[6],[8] Although all these studies have been conducted at almost the same time period when there was a surge of cases due to the Delta variant, there is a difference in vaccine effectiveness in these studies. This can be due to many factors, such as disease prevalence and vaccine coverage in different geographical areas. Furthermore, the participants of the study conducted at Puducherry included only the health-care workers, who were at a higher risk of infection. The fact that some (around 7%) of the participants of the study done at Vellore had received Covaxin, another COVID-19 vaccine available in India, is unlikely to have made any significant difference.

Various other studies from different parts of the world have reported the effectiveness of AstraZeneca, the Indian version of which is Covishield. The effectiveness of complete vaccination was reported to be 65.9% and 88% in cohort studies conducted in Chile and Scotland, respectively.[9],[10] The SARS-CoV-2 Delta variant has been reported to be 97% more transmissible as compared to the SARS-CoV-2 wild strain and has become the predominant variant of concern across the globe.[11]

We have used a test-negative, case‒control study design, which is the strength of our study. It is efficient and eliminates the bias arising due to differences in health-care-seeking behavior and access to health care for vaccination at the community level.[12] We minimized the potential bias of priority groups for vaccination like the elderly and people with comorbid conditions, by adjusting for age, gender, and comorbidities. In our study, patients with mild COVID-19 symptoms were in majority as compared to those with moderate-to-severe symptoms, which might have affected the true estimate of vaccine effectiveness against severe disease as shown by wide CIs. Further data may be required to improve confidence in this regard.

The limitation of our study is that it relies majorly on RT-PCR test reports. Hence, some people, despite being symptomatic for COVID-19, after taking the vaccination, might not have reported for testing believing that they were unlikely to develop COVID-19. Furthermore, we have not done genome sequencing, hence, were unable to predict the vaccine effectiveness separately for the wild strain and the mutant strains.


Our study showed that the ChAdOx1 nCoV-19 vaccine, either partial (single dose) or complete, was effective in reducing the risk of infection as well as the severity of the disease when the Delta variant had led to a rise in morbidity and mortality due to COVID-19.


We are thankful to the entire team of technicians, postgraduate students, and data management staff for their help during the study.

Financial support and sponsorship

We are thankful to Sharda University for its financial support.

Conflicts of interest

There are no conflicts of interest.


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