The effectiveness of COVID-19 vaccinations during Delta variant predominance

The current pandemic of coronavirus disease 2019 COVID-19 continues to cause havoc in the lives of people, despite the emergence of newer varieties of concern (VOCs). The causative pathogen, new severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is believed to have come out from a zoonotic source and rapidly spread in people through respiratory droplets as well as contact between people. Furthermore airborne transmission, particularly via aerosols, has been observed.

Study: Continuing Effectiveness of COVID-19 Vaccination in Urban Healthcare Workers during Delta Variant Predominance. Image Credit: EamesBot

The viruses are always evolving as changes to the genetic code are made during the replication of the genome. This leads to variations. A variant may have one or more mutations that differentiate it from other variants. Recently, concerns regarding the emergence of the delta variant have raised questions about the efficacy of COVID-19 vaccines against more recent VOCs. Data on COVID-19 vaccine effectiveness and the factors that determine the incidence of infection among healthcare workers during times when the delta variant is predominant are also scarce.

The study

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

The current study involved a COVID-19 vaccine program for employees in Massachusetts. Moderna was administered on December 23 in 2020, Pfizer on December 16 2020, Pfizer on December 16, 2020, and J&J/Janssen on February 20, 2021.

Vaccination was available to all workers , regardless of their working location from December 29 2020. A mandate was also announced on August 16, 2021 that required that all employees receive their final dose by the 18th of October 2021, with the exception of instances where a religious or medical exemption was granted.

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

The person-days at high risk for each healthcare professional were calculated and classified based on their vaccination status. The person-days of follow-up for healthcare workers were censored upon the conclusion of the study period or the date of their termination. This was the day they confirmed positive for COVID or when they received a third vaccination dose.


Overall there were 4,615 health care workers who had an average age of 45.0+-13.3 years (female dominance-76.0 percent) that contributed to 1,152.486 person-days at-risk during the study period. 45 percent of the study population was non-white. In the study, 4,418 (95.7 percent) had received at least one COVID-19 vaccine dose by the end of the study. The study included 58.3% who received Moderna, 39.4% Pfizer, 2.3% J&J/Janssen, and one (0.02 percent) who received a mix of Moderna and J&J/Janssen.

Multivariable adjustment revealed that the effectiveness of the vaccine was found to be 82.3% in fully vaccinated healthcare workers throughout the time period of study.

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. This allowed for an adjusted effectiveness of 76.5% to be deduced.

On examining 423 healthcare workers who contracted the disease prior to vaccination, the re-infection rate was nil. This depicted a 74,557 re-infection-free person-days, which began 10 days after the initial infection and ending at the date of receiving their first vaccine dose. Additionally, healthcare workers who were previously infected did not develop a breakthrough infection after vaccination.


This study is the first of its kind in the context of healthcare that predicts continued effectiveness of vaccines during the delta variant predominance. These results also show evidence of the natural acquisition of immunity. This study showed that vaccine effectiveness against the delta variant was 76%, which is almost identical to previous data that had demonstrated an average of 66 percent efficacy.

The strengths of this study were accounting for covariates and information biases such as background incidence and demographics as well as the inclusion of a multiethnic study population and uniform COVID-19 screening criteria and well-validated vaccination records. The study did not examine the effectiveness of specific vaccine manufacturers.

*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:

Content Source:

Gemma Wilson

Gemma is a journalism graduate with keen interest in covering business news – specifically startups. She has as a keen eye for technologies and has predicted quite a few successful startups over the last couple of years.

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