Effectiveness of COVID-19 vaccination during Delta variant predominance

The current pandemic caused by the coronavirus disease 2019 (COVID-19) continues to distort lives with the emergence of newer variants of concern (VOCs). The causative pathogen, the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is thought to have emerged from a zoonotic source and rapidly spread in humans through respiratory droplets and face-to-face contact. Additionally, airborne transmission, especially through aerosols, has been documented.

Study: Continued Effectiveness of COVID-19 Vaccination among Urban Healthcare Workers during Delta Variant Predominance. Image Credit: eamesBotStudy: Continued Effectiveness of COVID-19 Vaccination among Urban Healthcare Workers during Delta Variant Predominance. Image Credit: eamesBot

Such viruses continuously evolve as changes in the genetic code occur during replication of the genome, thus rendering variants. A variant harbors one or more mutations that differentiate it from other variants of the same virus. Recently, concerns regarding the emergence of the delta variant have questioned the effectiveness of COVID-19 vaccines against newer VOCs. Moreover, data on COVID-19 vaccine effectiveness (VE) and determinants of infection rates among healthcare workers during periods of delta variant predominance are limited.

The study

A new study published in medRxiv* preprint server investigated the continued effectiveness of COVID-19 vaccination during the delta variant predominance in a diverse and urban healthcare setting.

The present study entailed a COVID-19 vaccination program for employees undertaken in a community-based healthcare system in Massachusetts – with the Pfizer vaccine commencing on December 16, 2020; Moderna on December 23, 2020; and J&J/Janssen in February 2021.

Vaccination was available to all workers irrespective of their working location from December 29, 2020. Additionally, a mandate was announced on August 16, 2021, requiring all employees to receive their final dose by October 18, 2021, except in cases with an approved religious or medical exemption.

All actively serving healthcare workers were followed from December 16, 2020, to September 30, 2021, excepting those with prior COVID-19 infection from the main analyses. The primary outcome had a positive polymerase chain reaction (PCR) assay during the study period documented by the healthcare system’s Occupational Health department.

For each healthcare worker, the person-days at risk were calculated and categorized according to vaccination status. A healthcare worker’s follow-up person-days were censored at the end of the study period, or their termination date, the date tested positive for COVID, or the date they received a 3rd vaccine dose, whichever came first.


Overall, 4,615 healthcare workers, with an average age of 45.0±13.3 years (female predominance-76.0%), contributed to 1,152,486 person-days at risk during the study period. Forty-five percent of the study population was non-White. Among the participants, 4,418 (95.7%) had received at least one COVID-19 vaccine dose by the end of the study. Of these, 58.3% got Moderna; 39.4% Pfizer; 2.3% J&J/Janssen; and one (0.02%) got mixed doses of J&J/Janssen and Moderna.

On multivariable adjustment, the vaccine effectiveness was estimated to be 82.3% in fully vaccinated healthcare workers throughout the study period.

A secondary analysis limiting the study period based on the delta variant predominance in Massachusetts depicted an incidence rate of 5.8/10,000—which amounted to 15 events out of 25,910 person-days for unvaccinated person-days 1.3/10,000—amounting to 39 events out of 308,267 person-days, for 14 days after fully vaccinated. Through this, adjusted vaccine effectiveness of 76.5% was deduced.

On examining 423 healthcare workers who contracted the infection before vaccination, the re-infection rate was nil. This depicted a 74,557 re-infection-free person-days – initiating ten days after initial infection and censoring at the date of receiving their first vaccine dose. Additionally, previously infected healthcare workers did not experience any breakthrough infection events post-vaccination.


The present study is the first of its kind in healthcare settings projecting continued vaccine effectiveness during the delta variant predominance. The results also provide further evidence of naturally acquired immunity. In this study, the vaccine effectiveness was 76% against the delta variant, which was almost equivalent to previous data that projected a 66% efficacy.

The strengths of this study included accounting for covariates and information bias such as demographics and background incidence; inclusion of a multiethnic study population; consistent COVID-19 screening criteria; and well-validated vaccination records. The limitation of this study was that it did not examine individual manufacturers’ vaccine effectiveness.

*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:
Nidhi Saha

Written by

Nidhi Saha

I am a medical content writer and editor. My interests lie in public health awareness and medical communication. I have worked as a clinical dentist and as a consultant research writer in an Indian medical publishing house. It is my constant endeavor is to update knowledge on newer treatment modalities relating to various medical fields. I have also aided in proofreading and publication of manuscripts in accredited medical journals. I like to sketch, read and listen to music in my leisure time.


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