Fewer hospitalizations with omicron vs delta suggest reduced severity of omicron variant

Infection with the omicron variant of SARS-CoV-2 was less likely to result in hospitalization than infection with the delta variant, according to an observational study from Denmark presented at ECCMID 2022 and published in The Lancet.

“Our findings show that patients infected with the SARS-CoV-2 omicron variant were less likely to be admitted to hospital, with two-thirds of the risk observed in patients infected with the delta variant,” said the authors.

“[The results suggest] that omicron is associated with an inherently lower risk of severe disease … compared with delta,” they added.

For this study, the researchers used data of 188,980 individuals in Denmark with RT-PCR–confirmed SARS-CoV-2 infection between November 21* and December 19, 2021. Of these, 20.5 percent (n=38,669) were infected with the omicron variant (B.1.1.529) and 150,311 patients with the delta variant (B.1.617.2). About 2 percent had a prior SARS-CoV-2 infection. Almost 66 percent of patients (n=124,313) had received any dose of the vaccine, with a majority (86.6 percent) having received two doses and 4.3 percent a booster dose.

The overall hospitalization rate was 1.3 percent (n=2,435), of whom 9.1 percent (n=222) were infected with the omicron variant.

Patients infected with omicron were younger, had fewer comorbidities, and were more likely to have had a prior SARS-CoV-2 infection than those infected with delta. Among patients who had received 2–3 doses of the vaccine, more patients were infected with the omicron than delta variant, though the reverse was true in the unvaccinated population.

Overall, compared with patients who were unvaccinated or received only one vaccine dose, those who had received 2–3 doses had a lower risk of hospitalization** (adjusted risk ratio [adjRR], 0.24, 95 percent confidence interval [CI], 0.22–0.26). This reduced risk pertained to patients infected with either the omicron (adjRR, 0.29, 95 percent CI, 0.21–0.39) or delta variant (adjRR, 0.24, 95 percent CI, 0.22–0.27).

Infection with the omicron variant was less likely to warrant hospitalization compared with the delta variant (0.6 percent vs 1.5 percent; adjRR, 0.64, 95 percent CI, 0.56–0.75; p<0.0001). [ECCMID 2022, session SY114; Lancet Infect Dis 2022;doi:10.1016/S1473-3099(22)00154-2]

This reduction in hospitalization with the omicron vs delta variant was present regardless of whether patients had received three doses of the vaccine (RR, 0.50), two doses (RR, 0.71), or one or no doses (RR, 0.57; p_interaction=0.15).

“[V]accine effectiveness has been reported to decline at a faster rate for omicron compared with delta after the second vaccine dose,” said the authors. As such, the more marked reduction in hospitalization following one or no vaccine doses compared with two doses may be attributable to greater and longer protection against hospitalization after infection with the delta vs omicron variant after two doses, said the authors.   

Among hospitalized patients, those with omicron vs delta had a shorter duration of admission (median 15 vs 38 hours) and a lower risk of intensive care unit admission (2.7 percent vs 8.1 percent). Among patients hospitalized following infection with omicron, the risk of hospitalization was higher ≤4 months vs >4 months after the second vaccine dose (RR, 1.20 vs 0.59; p=0.0071), and lower ≤2 months vs >2 months after the third dose (RR, 0.43 vs 0.66; p=0.21).

“[Despite the findings,] even if the risk of hospitalization is lower for omicron compared with delta, the extensive spread due to higher transmissibility could rapidly outweigh any potential benefits of reduced severity and raise questions over whether omicron represents an important step towards the end of the pandemic,” the authors pointed out.

“Information on the documented severity of the omicron variant compared with the delta variant is crucial to understanding the public health consequences of the viral evolution of the omicron variant and to inform modelling studies and public health planning,” they concluded.