Severe outcomes more likely with Omicron XBB.1.16 in SG

In a preliminary analysis, the risk of severe outcomes was higher in individuals infected with the Omicron XBB.1.16 subvariant compared with those who were infected with other subvariants such as the XBB.1.5 or XBB.1.9.

“[T]he increase in COVID-19 cases in Singapore is now predominantly driven by four XBB subvariants emerging at different timepoints but presenting as one confluent outbreak that started in early March 2023,” said the investigators.

“In this study, we investigated outcomes of hospitalization and severe COVID-19 infection in individuals infected with different XBB subvariants, adjusted for potential confounders such as age and vaccination history,” they said.

From January 1 to May 27, 2023, whole genome sequencing was conducted on the respiratory samples of 7,733 COVID-19 cases (both local and imported) that was confirmed via PCR testing. Of these, 5,910 were infected with an XBB subvariant, the most dominant being XBB.1.9 (23 percent), followed by XBB.2.3 (22 percent), XBB.1.16 (20 percent), and XBB.1.5 (18 percent). The remaining 16 percent comprised other XBB sublineages. [Lancet Reg Health West Pac 2023;37:100849]

Despite the similarities in genetic profiles between XBB subvariants, XBB.1.16 has been designated as a variant of interest owing to its growth advantage and immune escape properties. [Lancet Infect Dis 2023;23:655-656; www.who.int/docs/default-source/coronaviruse/21042023xbb.1.16ra-v2.pdf?sfvrsn=84577350_1, accessed September 20, 2023]

The researchers found that the probability of severe outcomes was higher among those who were infected with XBB.1.16 as opposed to those infected with XBB.1.5 or XBB.1.9 subvariants after adjusting for confounders. No difference was seen in the comparison between XBB.1.16 and XBB.2.3.

“Furthermore, we observed no significant differences in the risk of COVID-19 hospitalization outcomes across different XBB subvariants,” they added.

Regardless of XBB subvariant, the risk of hospitalization was about 1.5 times higher among individuals who, at most, completed their primary vaccination series as opposed to those who had both the primary vaccine series and a booster.

Considering the potential incomplete vaccination history from the imported cases, a sensitivity analysis was conducted using local cases only. Similar findings were observed, the investigators noted.

Data monitoring still important despite the stepdown

“As countries scale down on COVID-19 testing and reporting, studies to distinguish the growth advantage of different COVID-19 variants would be increasingly challenging, given the drop in case ascertainment and lack of contact exposure data and studies on the impact of hybrid immunity on the risk of transmission,” the investigators pointed out. [Lancet Infect Dis 2022;22:1649]

Continued data collection on the clinical severity of cases and comparative evaluations remains imperative to help identify any changes in the severity of SARS-CoV-2 variants and factors contributing to the rise in COVID-19 hospitalizations, they continued.

“The findings of this study would also be useful for the development of vaccines,” they added.