The latest surge in COVID-19 cases in China is not surprising to researchers, who say that China will see an infection cycle every six months now that all COVID-19 restrictions have been removed and highly infectious variants are dominant. But they caution that rolling waves of infection carry the risk of new variants emerging.

“Unfortunately, a new reality with this virus [is that] we will have repeated infections,” says Ali Mokdad, an epidemiologist at the Institute for Health Metrics and Evaluation at the University of Washington in Seattle. “The fear is that this virus will produce a new variant that can compete with the current ones and is more severe.”

The current surge is caused mainly by a highly infectious subvariant of Omicron called XBB.1.5, first identified in India last August. According to Nanshan Zhong, a prominent respiratory physician in China, as many as 65 million people could become infected per week by the end this month.

This is the first major reinfection wave that China has seen since the central government dropped all its COVID-19 control measures in December, prompting a widespread Omicron outbreak.

China has vaccinated more than 90% of its population, and the outbreak in December infected at least 85% of its people, says Zhong. But immunity is waning, and XBB can evade protection from vaccines and prior infections. Mokdad says that, although XBB has not caused a major rise in hospitalizations and deaths, the sheer number of infections could put pressure on China’s health-care system.

XBB is also causing minor waves in other parts of the world, such as Singapore and the United States.

“This is what we see everywhere, but with a large population such as China, it is more apparent,” says Mokdad.

Yunlong Cao, an immunologist at Peking University in Beijing, and his team have found that antibodies generated against Omicron variants BA.5 and BF.7, the dominant strains during the December wave in China, can provide about four months of protection against strains such as XBB1.

Kayoko Shioda, an epidemiologist at Boston University in Massachusetts, says that previous COVID-19 surges in other countries have shown that XBB is more transmissible than earlier forms. “Once it enters the population, XBB spreads and becomes the predominant variant much quicker than other variants,” she says.

Last December, more than 200 million people in China contracted COVID-19 in 20 days. This time, the wave is spanning several months, owing to the differences in people’s immune backgrounds, such as antibody levels, says Cao. “The peak of COVID-19 waves will generally become flatter and more stretched out after each cycle, which is a pattern we see in countries like the US. People are still getting infected in the US, just not all at once,” he adds. A flatter wave would also lessen the burden on health-care systems, Cao says.

New boosters

Because China no longer publishes its COVID-19 case count, it is unclear how many people are becoming infected in the latest wave; however, the Beijing health authority says the number of COVID-19 cases reported in the capital city quadrupled between late March and mid-April. Cao says it’s hard to make estimates without accurate data. But on the basis of his past research, he estimates that at least 30% of the population could become reinfected in this wave, amounting to more than 400 million people.

Scientists say having a good surveillance system to monitor and track emerging virus variants is very important, given that infection cycles will continue to happen. A new variant that could supersede current ones remains a concern, says Mokdad. “Imagine a Delta type of variant with XBB capacity of spreading. This will cause us a lot of damage.”

Tracking the virus’s evolution also means that scientists can update booster vaccines accordingly.

Facing the ongoing COVID-19 wave, several major Chinese cities, including Beijing and Shanghai, have started to inoculate residents with a quadrivalent COVID-19 booster, made by the Chinese biotechnology company Sinocelltech Group. The vaccine, first approved for emergency use in March, is designed to provide broad-spectrum protection against the Alpha, Beta, Delta and Omicron BA.1 coronavirus variants. Sinocelltech announced last month that one booster shot using this jab can prevent 82% of SARS-CoV-2 infections — including ones caused by XBB — for up to four months. The final results from the vaccine’s clinical trial are not yet available.

This week, Chinese state media also announced a new recombinant trivalent vaccine that targets XBB, BA.5 and Delta was approved for emergency use. Describing it as the world's first approved vaccine against XBB, the report said the vaccine will be rolled out within two months.

Although current vaccines can provide good protection against severe disease and death, they are not very good at providing long-lasting protection against infection. “But that doesn’t mean we should just give up. Repeated infections, even with a mild virus like XBB, can still lead to health problems like long COVID. Vulnerable people, such as older adults, are still at risk of getting very ill,” Cao says.