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January 18, 2022

A recent article in the New England Journal of Medicine (NEJM) presents results from a large cohort study from New York State on COVID-19 vaccine effectiveness.  They used data for 8,690,825 adults in New York State to assess the effectiveness of the BNT162b2, mRNA-1273, and Ad26.COV2.S vaccines against laboratory-confirmed Covid-19 and hospitalization with Covid-19 (i.e., Covid-19 diagnosed at or after admission). They then compared cohorts defined according to vaccine product received, age, and month of full vaccination with age-specific unvaccinated cohorts by using linked statewide testing, hospital, and vaccine registry databases. They then assessed vaccine effectiveness against Covid-19 in two separate time periods: May 1 through September 3, 2021, and against hospitalization with Covid-19 from May 1 through August 31, 2021

The authors discuss conducting a person level analysis of all the cases in the 1st time interval and calculating hazard ratios and associated 95% confidence intervals for each product and age group in order to compare the vaccinated to age-specific unvaccinated cohorts.  Then as common practice calculating vaccine effectiveness as 1 minus the relative risk. However, they took 1 – the hazard ratio and treated it like a relative risk

In their 2nd time frame, they used unit of observation as admission rather than an individual patient using an aggregate rates approach (??).  Again, for each product and age cohort they calculated incidence rates of new admission along with 95% Poisson confidence intervals and compared those between vaccinated and unvaccinated cohorts. This time they calculated vaccine effectiveness as 1 minus the incidence rate ratio.

Trends in vaccine effectiveness against Covid-19 were compared with the percentage of delta-variant specimens from the CDC national SARS-CoV-2 genomic surveillance program for the Department of Health and Human Services region containing New York State.5 The association between vaccine effectiveness and delta-variant prevalence was summarized each week with the use of the Pearson correlation coefficient (r), after application of the logit transformation.


In their results they used there were 150,865 cases of Covid-19 and 14,477 hospitalizations with Covid-19.  According to their results, During the week of May 1, 2021, when the delta variant made up 1.8% of the circulating variants, the median vaccine effectiveness against Covid-19 was 91.3% (range, 84.1 to 97.0) for BNT162b2, 96.9% (range, 93.7 to 98.0) for mRNA-1273, and 86.6% (range, 77.8 to 89.7) for Ad26.COV2.S. Thereafter, effectiveness declined in all cohorts, from a median of 93.4% (range, 77.8 to 98.0) during the week of May 1 to peak point of 73.5% (range, 13.8 to 90.0) around July 10, when the prevalence of the delta variant was 85.3%. By the week of August 28, when the prevalence of the delta variant was 99.6%, the effectiveness was 74.2% (range, 63.4 to 86.8).

Effectiveness against hospitalization with Covid-19 among adults 18 to 64 years of age remained almost consistently greater than 86%, with no apparent time trend. The effectiveness had declined from May through August among persons 65 years of age or older who had received BNT162b2 (from 94.8 to 88.6%) or mRNA-1273 (from 97.1 to 93.7%). The effectiveness of Ad26.COV2.S was lower than that of the other vaccines, with no trend observed over time (range, 80.0 to 90.6%).

Their conclusions were that the effectiveness of the three vaccines against Covid-19 declined after the delta variant became predominant. The effectiveness against hospitalization remained high, with modest declines limited to BNT162b2 and mRNA-1273 recipients 65 years of age or older.

Written by,

Usha Govindarajulu


COVID-19, delta variant, vaccine effectiveness, hazards, biostatistics



Rosenberg ES, Dorabawila, V, Easton D, Bauer UE, Kumar J, Hoen R, Hoefer D, Wu M, Lutterloh E, Conroy MB, Greene D, and Zuker HA. (2022). “Covid-19 Vaccine Effectiveness in New York State” N Engl J Med; 386: 116-127.