A higher proportion of adults hospitalized with COVID-19 during the period of omicron dominance were fully vaccinated compared to the period of delta dominance, according to a new analysis, but those people were also less likely to be severely ill or die.
The Centers for Disease Control and Prevention (CDC) analysis of a single hospital in Los Angeles found there were more hospitalizations during omicron compared to delta, but that among omicron-period patients, vaccination — particularly vaccination plus a booster dose — was associated with lower likelihood of admission to an intensive care unit.
“COVID-19 vaccination, particularly a booster dose, continues to be critical in mitigating the health care burden of the omicron variant,” the CDC concluded in its Morbidity and Mortality Weekly Report.
Among adults at least 65 years old hospitalized during omicron, vaccination was also associated with a lower likelihood of death while hospitalized.
Omicron-period hospitalizations overall were associated with a lower likelihood of ICU admission, mechanical ventilation and death while hospitalized, compared with delta-period hospitalizations.
However, the proportion requiring ICU admission and ventilation did not differ significantly when broken down by vaccination status, suggesting that much of the lower disease severity observed during omicron predominance might be driven by increased population-level vaccine-conferred immunity, the CDC said.
From mid-July through mid-December, the proportion of fully vaccinated adults in Los Angeles County increased nearly 20 percent, from approximately 65 percent to 77 percent, but the proportion of COVID-19 hospitalizations occurring in fully vaccinated adults increased almost 60 percent, from approximately 25 percent to 40 percent.
According to the CDC, the increases in infections among vaccinated persons during the period of omicron predominance were likely driven both by waning vaccine-derived immunity over time and by relative resistance to vaccine neutralization in the omicron variant compared with the delta variant.
There were 339 adults hospitalized during the delta predominant period of July 15 to Sept. 23, and 737 adults hospitalized during the omicron period of Dec. 21 through Jan. 27.
The CDC also noted that while some reports indicated omicron was causing less severe illness compared to delta, most patients hospitalized during the early omicron period still suffered from severe lower respiratory illness.
The agency also addressed the distinction between people admitted for COVID-19 and those diagnosed with COVID-19 after admission.
“Approximately 20 percent of SARS-CoV-2 admissions during early Omicron predominance were likely for reasons other than COVID-19, a proportion even higher among young and vaccinated adults. Given high rates of SARS-CoV-2 community transmission, this is not unexpected,” the CDC said.
One-third of patients classified as having been admitted for COVID-19 received no COVID-19–directed therapies.
“However, the pandemic health care burden is not limited to hospitalizations for symptomatic COVID-19,” the CDC noted in the study. “Even patients with positive SARS-CoV-2 test results admitted for non-COVID-19 conditions require isolation rooms and use of personal protective equipment and might transmit infection to health care workers, exacerbating staff shortages.”