How effective is the Johnson & Johnson COVID-19 vaccine?

The coronavirus disease 2019 (COVID-19) continues to pose a major threat to global health, particularly due to the continuous emergence of new severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants. This lends urgency to the need for a safe and effective vaccine that can protect against SARS-CoV-2 infection.

The Janssen vaccine is an adenovirus-vector vaccine that is designed to provide single-dose immunity against COVID-19. Earlier trials have shown its efficacy under randomized controlled trial conditions. Recently, a new study published on the preprint server medRxiv* demonstrates the real-world efficacy of this vaccine, its stability over time, as well as its efficacy against the SARS-CoV-2 Delta variant.

Study: Effectiveness of the Single-Dose Ad26.COV2.S COVID Vaccine. Image Credit: Lutsenko_Oleksandr /


Vaccine efficacy (VE) depends on the virus variant in current circulation. The SARS-CoV-2 Delta variant is known to be highly transmissible and resistant to neutralization by wild-type spike-elicited antibodies.

The durability of protection over time may change, as may the phenotype of the disease itself. Large health databases, such as those of healthcare insurance providers, may be mined to provide a wealth of information on VE, long-term protection, and clinical features of COVID-19, much faster than a trial set up de novo.

The current study used national medical claims data in the United States to determine the VE of the Ad26.COV2.S vaccine against breakthrough infections and hospitalizations related to SARS-CoV-2 between March and July 2021. The researchers were particularly interested in four American states where the Delta variant was circulating at high levels, which included Florida, Louisiana, Arkansas, and Missouri.

The researchers point out that a large proportion of COVID-19 vaccinations came via routes other than that of the healthcare providers, such as employers, pharmacies, and mass vaccination sites, where insurance claims were not filed routinely. The U.S. Centers for Disease Control and Prevalence estimate that 57% of Americans older than 12 years were vaccinated by July 12, 2021. However, only 34% of these names showed up on healthcare claims, thus indicating that many vaccinations are going unrecorded in insurance claims.

This affects the final estimates of VE based on vaccinated as compared to unvaccinated cohorts as per insurance claims, as the control or ‘unvaccinated’ group is comprised of both vaccinated and unvaccinated individuals. Taken together, this gap in data will cause the VE to be falsely low. The researchers corrected for this by adding a correction factor of 40% to compensate for this under-recording.

Study findings

The investigators included approximately 390,000 vaccinated individuals and over 1.5 million matched controls in the study. In the high-Delta states, over 29,000 vaccinated individuals were matched against about 110,000 controls.

Follow-up was completed in almost all the vaccinated and 82% of controls, with the missing 18% in the unvaccinated group being removed because they took the vaccine during the study period.

Nationally, the incidence of COVID-19 in the vaccinated group was 12/1,000 person-years (PY), which was comparable to 39.5 for every 1,000 individuals in the unvaccinated cohort. Taken together, this provides a VE of 79% against detected COVID-19.

Hospitalizations were estimated at 2.4/1,000 PY and 9.1/1,000 PY in the vaccinated and unvaccinated cohorts, respectively, with the VE being 81%. Among those aged 50 or older, VE was 75% as compared to 83% in those younger than 50. VE was lowest at 64% in the immunocompromised.

In the high-Delta states, COVID-19 incidences were 19.4/1,000 PY and 61.4/1,000 PY in the vaccinated versus unvaccinated groups, respectively. This led the VE in these states to be 79% in June and 78% in July, both of which were the peak Delta incidence months in these states. The VE for hospitalizations was 83% overall but rose to 85% in June and July. The VE among those over and below 50 years were 84% and 81%, respectively.

Duration of VE

The VE remained stable for 14 days post-vaccination to up to 152 days, from 81% in May to 77% in July, when the Delta variant was rising rapidly to dominance. No significant reduction was observed in the high-Delta states.


The single-dose Ad26.COV2.S vaccine showed a high degree of protection against COVID-19 incidence and hospitalization, which remained stable over the study period, including the peak of the delta wave in high-prevalence states.

These results provide evidence that the VE observed in the ENSEMBLE trial translates into clinical practice, lasts over at least 152 days post-vaccination, and remains effective amid high Delta variant incidence.”

The VE specific to each variant was not calculated, as the Data did not provide details on viral sequences. As expected, there was a slight decline in VE with age, though it remained high.

Even without the correction made for under-recording, the falsely low VE would remain high at 69% against infection and 73% for hospitalizations. Importantly, since many infections go unrecorded, this would not affect the VE for COVID-19-related hospitalizations. However, these unreported infectious would underestimate the VE for infections.

The study thus demonstrates the high and stable VE of a single dose of Ad26.COV2.S in high-risk patients and high-Delta-incidence areas. It also indicates the utility of using claims databases to provide timely information to shape policies and healthcare guidelines.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
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