A High Transmission Rate of Omicron BA.5 in China, Despite High Vaccine Coverage

Image courtesy of Adobe Stock/ Aleksandar.

The COVID-19 Omicron variant was first identified by South African scientists in late 2021, and in the years since, Omicron and its subvariants have reigned supreme. In 2022, Urumqi, China experienced a severe COVID-19 outbreak due to the Omicron BA.5 variant.

The emergence of each new and highly infectious variant raises concerns about whether COVID-19 vaccines will remain effective. A recent study, published in JAMA Network Open, examined the transmission characteristics of COVID-19 Omicron BA.5 variants, as well as whether vaccine were associated with a protective outcome against infection with these variants.

This retrospective cohort study was conducted with data from an Omicron BA.5 outbreak in Urumqi from August 7-September 7, 2022. China previously had a “zero COVID-19 policy,” strict regulations which resulted in very low levels of COVID-19 compared to the majority of the world. Thus, this study was among the first to estimate the real-world effectiveness of COVID-19 vaccines against emerging variants in China.

After analyzing detailed contact-tracing data collected during Omicron BA.5 outbreaks in Urumqi, China, the investigators calculated epidemiological features, infectivity, and vaccine effectiveness against Omicron BA.5 transmission.

Participants included all individuals with confirmed COVID-19, as well as their close contacts, identified by the disease surveillance program of the Xinjiang Uygur Autonomous Region Health Committee. Contact-tracing data were obtained and analyzed as part of an ongoing public health investigation.

For each infected individual, the investigators collected information on age, sex, exposure history, contact setting (household or nonhousehold), symptom onset date, date of diagnosis, and vaccination record. The investigators compared the protective effects of a booster dose versus 2 doses of inactivated vaccine.

A total of 1139 participants were diagnosed with COVID-19, a cohort that averaged 55.3% female and 37.4 years of age. In addition, 51323 close contacts who tested negative for COVID-19 were included, averaging 51.2% female and 38.4 years of age.

The mean generation interval was 2.8 days, viral shedding period was 6.7 days, and incubation period was 5.7 days. Among the 1139 infected participants, 86.0% (n = 980) received at least 2 doses of a COVID-19 vaccine. However, despite this high vaccination rate and intensive contact tracing and infection control protocols, an Omicron BA.5 outbreak occurred in Urumqi, China.

High transmission risks were identified in household settings (14.7% secondary attack rate), and both younger (0-15 years; 2.5% secondary attack rate) and older (> 65 years; 2.2% secondary attack rate) age groups. Vaccine effectiveness against Omicron BA.5 infection for a 3-dose vaccine regimen versus a 2-dose primary series was 28.9%. Vaccine efficacy against BA.5 transmission remained high in the 15-90 days after receiving a booster dose, at 48.5%.

“These findings suggest the importance of continuously evaluating vaccine effectiveness against emerging SARS-CoV-2 variants,” the study authors wrote. Their findings present key transmission characteristics of COVID-19 as it evolves, as well as vaccine efficacy against these new and emerging variants.