Differences in symptoms and risk of hospitalization following infection with Omicron or Delta SARS-CoV-2 variants among vaccinated people

By Bhavana KunkalikarReviewed by Danielle Ellis, B.Sc.Apr 12 2022

A recent study published in The Lancet* assessed the prevalence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) symptoms and the risk of hospitalization in vaccinated individuals infected with SARS-CoV-2 Omicron and Delta variants.

Various studies have reported the risk of hospitalization in SARS-CoV-2 Omicron-infected patients. However, extensive research is needed to ascertain the coronavirus disease 2019 (COVID-19) symptom prevalence and related hospitalization in Delta-infected individuals.    

About the study

In the present prospective longitudinal large-scale-observational study, the researchers quantified the differences between prevalent symptoms, risk of hospitalization, and duration of infection in SARS-CoV-2 Omicron and Delta variant-infected vaccinated individuals.

The team used the ZOE COVID study application that captured self-reported COVID-19 data. The participants provided baseline information regarding their health and demographics and reported daily updates associated with any COVID-19-related symptoms experienced, results of COVID-19 tests taken, vaccines received, and any relevant outcomes.

Participants eligible for the study were aged between 16 and 99 years, were residents of the United Kingdom (UK), and had a body-mass index (BMI) between 15 and 55 kg/m². These individuals were also vaccinated with at least two doses of any COVID-19 vaccine and tested SARS-CoV-2 positive by polymerase chain reaction (PCR) or lateral flow antigen test (LFAT) between June 1^st, 2021, and January 17^th, 2022.

The primary outcomes of the study assessed were the possibility of a participant developing a particular symptom before or within seven days of testing positive for either SARS-CoV-2 Delta or Omicron variants. The team also assessed the likelihood of infected participants reporting symptoms like fever, cough, or loss of smell and the chances of patient hospitalization within the study period. The secondary outcome assessed was the duration of symptoms for the Delta and Omicron infections. The team defined acute symptom duration as the period between COVID-19 symptom onset and symptom recovery.         

Results

The study results showed that 63,002 participants were SARS-CoV-2-positive by either PCR or LFAT during the study period. Approximately 64% and 63% of the SARS-CoV-2 Omicron and Delta-infected participants, respectively, were females, among which 33,785 and 29,217 participants were infected with the SARS-CoV-2 Delta and Omicron variants, respectively.

Out of the participants that reported one or more potential COVID-19 symptoms, 81.6% had a runny nose, 77.9% had a headache, 70.7% reported sneezing, 60.8% had a sore throat, and 52.7% experienced a loss of smell. Among the symptomatic Omicron-infected participants, 76.5% had a runny nose, 74.7% had a headache, 70.5% had a sore throat, 63.0% reported sneezing, 49.8% experienced persistent cough, and 42.6% reported hoarseness of voice. Notably, the average number of symptoms reported by the Omicron-infected group was lower than that of the Delta-infected group.  

The assessment of symptom prevalence showed that 12 symptoms, including loss of smell, altered sense of smell, sneezing, runny nose, brain fog, eye soreness, headache, fever, hair loss, blistering on feet, ear ringing, and dizziness, were significantly less prevalent in Omicron patients than in the Delta patients. However, symptoms like sore throat and hoarseness of voice had a higher likelihood of being reported by Omicron patients than Delta patients. Furthermore, the study showed that Omicron patients were less likely to experience fever, cough, or loss of smell as compared to the Delta patients.

The study also observed that the risk of hospitalization among participants who were vaccinated with two or three doses was 1.9% in the Omicron-infected and 2.6% in the Delta-infected patients. Moreover, the average acute symptom duration was 8.89 days in Delta-infected patients, while that in Omicron-infected patients was 6.87 days. Notably, the average acute symptom duration in individuals vaccinated with two and three doses was 9.57 and 7.71 days in the Delta and 8.30 and 4.4 days in the Omicron patients, respectively. The study also found that Omicron patients were twice more likely to report symptom recovery within a week of symptom onset as compared to Delta patients.             

Conclusion

The study findings showed that among individuals vaccinated against COVID-19, the symptoms experienced by SARS-CoV-2 Omicron-infected patients were significantly milder and lasted for a shorter duration than those of Delta-infected patients. Furthermore, loss of smell, a symptom considered characteristic of COVID-19, was significantly lesser by Omicron-infected than by Delta-infected patients.