Association of mRNA Vaccination With Clinical and Virologic Features of COVID-19 Among US Essential and Frontline Workers

Abstract

Importance: Data on the epidemiology of mild to moderately severe COVID-19 are needed to inform public health guidance.

Objective: To evaluate associations between 2 or 3 doses of mRNA COVID-19 vaccine and attenuation of symptoms and viral RNA load across SARS-CoV-2 viral lineages.

Design, setting, and participants: A prospective cohort study of essential and frontline workers in Arizona, Florida, Minnesota, Oregon, Texas, and Utah with COVID-19 infection confirmed by reverse transcriptase-polymerase chain reaction testing and lineage classified by whole genome sequencing of specimens self-collected weekly and at COVID-19 illness symptom onset. This analysis was conducted among 1199 participants with SARS-CoV-2 from December 14, 2020, to April 19, 2022, with follow-up until May 9, 2022, reported.

Exposures: SARS-CoV-2 lineage (origin strain, Delta variant, Omicron variant) and COVID-19 vaccination status.

Main outcomes and measures: Clinical outcomes included presence of symptoms, specific symptoms (including fever or chills), illness duration, and medical care seeking. Virologic outcomes included viral load by quantitative reverse transcriptase-polymerase chain reaction testing along with viral viability.

Results: Among 1199 participants with COVID-19 infection (714 [59.5%] women; median age, 41 years), 14.0% were infected with the origin strain, 24.0% with the Delta variant, and 62.0% with the Omicron variant. Participants vaccinated with the second vaccine dose 14 to 149 days before Delta infection were significantly less likely to be symptomatic compared with unvaccinated participants (21/27 [77.8%] vs 74/77 [96.1%]; OR, 0.13 [95% CI, 0-0.6]) and, when symptomatic, those vaccinated with the third dose 7 to 149 days before infection were significantly less likely to report fever or chills (5/13 [38.5%] vs 62/73 [84.9%]; OR, 0.07 [95% CI, 0.0-0.3]) and reported significantly fewer days of symptoms (10.2 vs 16.4; difference, -6.1 [95% CI, -11.8 to -0.4] days). Among those with Omicron infection, the risk of symptomatic infection did not differ significantly for the 2-dose vaccination status vs unvaccinated status and was significantly higher for the 3-dose recipients vs those who were unvaccinated (327/370 [88.4%] vs 85/107 [79.4%]; OR, 2.0 [95% CI, 1.1-3.5]). Among symptomatic Omicron infections, those vaccinated with the third dose 7 to 149 days before infection compared with those who were unvaccinated were significantly less likely to report fever or chills (160/311 [51.5%] vs 64/81 [79.0%]; OR, 0.25 [95% CI, 0.1-0.5]) or seek medical care (45/308 [14.6%] vs 20/81 [24.7%]; OR, 0.45 [95% CI, 0.2-0.9]). Participants with Delta and Omicron infections who received the second dose 14 to 149 days before infection had a significantly lower mean viral load compared with unvaccinated participants (3 vs 4.1 log10 copies/μL; difference, -1.0 [95% CI, -1.7 to -0.2] for Delta and 2.8 vs 3.5 log10 copies/μL, difference, -1.0 [95% CI, -1.7 to -0.3] for Omicron).

Conclusions and relevance: In a cohort of US essential and frontline workers with SARS-CoV-2 infections, recent vaccination with 2 or 3 mRNA vaccine doses less than 150 days before infection with Delta or Omicron variants, compared with being unvaccinated, was associated with attenuated symptoms, duration of illness, medical care seeking, or viral load for some comparisons, although the precision and statistical significance of specific estimates varied.

Figure 1.. Viral Load and Plaque-Forming Units of SARS-CoV-2 Infection by Virus Lineage, Vaccination Status, and COVID-19 Characteristics

Viral load was measured by quantitative reverse transcriptase–polymerase chain reaction assay to assess viral shedding. Plaque-forming units were quantified on Vero cells ectopically expressing transmembrane serine protease 3 and human angiotensin converting enzyme 2 to assess virus viability. ^aMean difference and 95% CIs were estimated using linear regression controlling for time from index to swab collection, number of daily medications reported, and race/ethnicity. Hours of missed work is additionally adjusted for occupation.

Figure 2.. Estimated Viral Load Over the Course of SARS-CoV-2 Infection for Both Vaccinated and Unvaccinated Participants by Virus Lineage and Symptomatic vs Asymptomatic Infections

Viral load over the time course of infection for vaccinated and unvaccinated participants with at least 3 specimens. Index date is defined as the date of illness onset or the date of specimen collection if the infection was asymptomatic. Negative days indicate infection prior to symptom start. Points are individual positive reverse–transcriptase polymerase chain reaction specimens. Specimens below 1 on the y-axis represent a viral load below the minimum quantifiable level and estimated from cycle threshold values. 10⁶ is the maximum quantifiable by the assay. Curves are predicted mean viral load from bayesian linear models with time from index as a 3-knot natural spline. A, Overall viral count of origin strains and Delta variants are higher than Omicron variants, with mean differences of 0.26 (95% CI, 0.0-0.5) and 0.21 (95% CI, 0.0-0.4), respectively. B. Symptomatic COVID-19 infections had significantly higher viral count than asymptomatic infections (mean difference, 0.70 [95% CI, 0.4-1.1]).