In a recent study published in the Journal of the American Medical Association, researchers investigated the association between two or three doses of the messenger ribonucleic acid (mRNA) coronavirus disease 2019 (COVID-19) vaccine and viral loads and symptoms during infections with different severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants.

Background

Community cohort studies reported a significant decrease in the severity of COVID-19 symptoms, viral shedding, and duration of infection after widespread COVID-9 vaccination efforts. However, the emergence of immune-evading SARS-CoV-2 variants of concern and the waning of vaccine-induced immunity is increasing breakthrough infections, albeit with mild symptoms.

Mild or moderate COVID-19 severity is observed in most SARS-CoV-2 infections, and such cases are thought to contribute significantly to the spread of COVID-19. However, the data on such cases, as well as information on routine community testing and the presence of multiple SARS-CoV-2 variants in a population, is limited. Such data are essential to understand the efficacy of vaccinations against the severity of infections from SARS-CoV-2 variants.

About the study

In the present study, the researchers used a prospective cohort of frontline and essential workers in six states in the United States (U.S.) to study the virological and clinical characteristics of SARS-CoV-2 infections from the original strain and Delta and Omicron lineages and compare clinical outcomes and viral loads. The study also investigated the association between two and three mRNA vaccine doses and COVID-19 symptoms, severity, and viral load.

Frontline or essential workers as individuals whose occupation requires 20 hours or more of regular contact with others, such as those working in the education, medical, waste management, and transportation sectors. The data comprised sociodemographic characteristics, SARS-CoV-2 infection history, and chronic medical conditions. Vaccination cards and online surveys were used to gather information about COVID-19 vaccination status.

Nasal swabs were collected from the participants every week and tested using reverse transcriptase–polymerase chain reaction (RT-PCR). Whole genome sequencing was used to identify the viral lineage, and quantitative RT-PCR was performed to assess viral loads. For samples with a cycle threshold value greater than 30 with identified viral lineages, plaque-forming units (PFU) on Vero cells were used to determine the viability of the virus.

The clinical outcomes measured in the study were the presence and number of COVID-19 symptoms, duration of the infection, days spent in bed for at least half a day, number of days away from work, and the medical care requirements. The virological outcomes measured were PFU counts and viral loads.

Results

The results reported that of the 1199 COVID-19 infections in the cohort, the percentages of infections from the original strain, Delta variant, and Omicron variant were 14%, 24%, and 62%, respectively. The severity of symptoms was correlated to the vaccination doses.

Individuals with two doses of the vaccine were less symptomatic during Delta infections than unvaccinated individuals. A third vaccine dose one to 21 weeks before infection significantly reduced the incidence of fever and chills and the duration of the symptoms.

The severity of symptoms during infections with the Omicron variant did not vary much between individuals with two vaccine doses and no vaccination. Still, individuals with three vaccine doses were significantly less likely to experience fever and chills or require medical attention than unvaccinated individuals.

The virological outcomes indicated that individuals who contracted the Delta or Omicron infection two to 21 weeks after the second vaccination dose had significantly lower viral loads than unvaccinated individuals. The viral loads during Omicron BA.1 infections were higher than during infections with the original strain and similar to the Delta infection viral loads. The durations of Omicron infections were shorter, and the symptoms were milder.

The authors believe that the higher viral load combined with the high frequency of mild or asymptomatic cases could explain the increased transmissibility observed during the Omicron prevalence period.

Conclusions

Overall, the study indicated that two or three doses of mRNA vaccines less than 150 days before contracting a SARS-CoV-2 infection significantly reduced the severity, duration, and viral load of Delta or Omicron infections in frontline workers. The vaccine doses also reduced the need to seek medical care.

According to the authors, the high transmissibility of Omicron variants could be related to the higher viral loads and milder symptoms during Omicron infections as compared to infections with the original strain.

Written by

Dr. Chinta Sidharthan

Chinta Sidharthan is a writer based in Bangalore, India. Her academic background is in evolutionary biology and genetics, and she has extensive experience in scientific research, teaching, science writing, and herpetology. Chinta holds a Ph.D. in evolutionary biology from the Indian Institute of Science and is passionate about science education, writing, animals, wildlife, and conservation. For her doctoral research, she explored the origins and diversification of blindsnakes in India, as a part of which she did extensive fieldwork in the jungles of southern India. She has received the Canadian Governor General’s bronze medal and Bangalore University gold medal for academic excellence and published her research in high-impact journals.