The Omicron variant of the coronavirus accounted for 90% of cases at one Houston hospital system by mid-December, but patients are not as severely ill as those infected by previous variants, a team of doctors reported Monday.
And while nearly half of all patients the doctors saw had been vaccinated, only 10% had gotten booster shots – which supports the value of boosters in fighting Omicron.
Dr. James Musser of the Houston Methodist Research Institute and colleagues reported on the cases of 862 Covid-19 patients treated at the hospital system between November 27 and December 18. The system sequences the genomes of most cases, so they have a good picture of which variants are involved.
“In three weeks Omicron spread throughout the Houston metropolitan region and became the cause of 90% of new COVID-19 cases,” they wrote in their report, posted online as a preprint and not yet peer-reviewed.
“Compared to patients infected with either Alpha or Delta variants and cared for in our system, significantly fewer Omicron patients were hospitalized, and those who were hospitalized required significantly less intense respiratory support and had a shorter length of stay. We cautiously interpret our findings to be consistent with decreased disease severity among Houston Methodist Omicron patients,” they added.
Remember: It’s not clear that Omicron is less virulent, and the study was not designed to show that, they cautioned. “Many factors undoubtedly have contributed, including but not limited to increased vaccination uptake, population immunity, and patient demographics such as younger age. The extent to which our findings translate to other cities and other patient populations, including children, is unknown.”
But they could show how quickly Omicron took over. “The estimated case doubling time during this three-week period was approximately 2.2 days, which means that Omicron increased in frequency approximately three times faster than Delta had increased in our area, an unprecedented trajectory for SARS-COV-2 infections,” they wrote.
And they could show that Omicron evades the protection offered by vaccines, at least to some degree. “We found 430 of the 862 total Omicron patients (49.9%) for whom we have whole genome sequence data met the CDC definition of vaccine breakthrough cases,” they wrote. Just 9.9% of the patients had gotten a booster dose, they found.
The researchers could not find an easy way to measure whether the time since a person was vaccinated had an effect on their risk of a breakthrough infection. They also noted that their study represents just 5% of cases in the Houston metro region.