Study: Omicron variant may be milder, send fewer to hospital

JOHANNESBURG — An early study of coronavirus test results in South Africa suggests, so far, patients infected with the omicron variant may be hospitalized less often than patients infected with earlier versions of the virus.

The study — which was released Tuesday and is based on only three weeks of data — also shows vaccines are not as effective against the variant, which poses a higher risk of breakthrough infections.

Public health researchers have cautioned that data from a few more weeks will be needed to draw firmer conclusions, in part because omicron has not yet spread widely and because only a small percentage of infected people become ill enough to be hospitalized.

The study, by a private health insurance company, offers a preliminary look at the course of the omicron variant, but there are other possible explanations for the trends that were observed.

For example, infections may appear to be milder overall because more people in the current wave have some protection from prior infection or immunization. Moreover, the mean age of the people in the study was 34, and young people generally tend to have mild symptoms. That may also make omicron infections appear milder than they really are. But the findings do echo other research showing that while new coronavirus cases have increased exponentially, the trajectory of hospital admissions has been much flatter.

A snapshot of the first three weeks of each of the four waves of infection in South Africa shows hospital admissions are significantly lower during the omicron-driven fourth wave — 38 admissions per 1,000, compared with 101 per 1,000 during the delta-driven wave, and 131 per 1,000 when the beta variant was dominant, the study showed.

Anecdotally, those who were admitted to hospitals had much milder illness and shorter hospital stays, according to accounts physicians shared with researchers. A majority of patients who needed oxygen were unvaccinated, and only 16 percent of ICU admissions were vaccinated.

The efficacy of the Pfizer vaccine seemed to wane, decreasing immunity to just 33 percent during the omicron wave from 80 percent in September and October after the delta wave had subsided, the study found.

“Omicron has materially reduced vaccine effectiveness against new infections, potentially compounded by waning durability,” the authors concluded. Booster shots increased the vaccine’s efficacy against infection. Still, the Pfizer vaccine’s efficacy against severe illness and hospitalization was about 70 percent after two doses, the study found.

The study also found an increased risk of reinfection with the omicron variant, and waning immunity from previous infections. People who were infected with the delta variant of the coronavirus had a 40 percent relative risk of contracting the omicron variant, while those infected during the beta-driven wave at the beginning of 2020 faced a 60 percent chance of reinfection with omicron.