(Bloomberg) -- A recent Covid-19 outbreak that started at an airport in the eastern city of Nanjing is testing China’s aggressive containment efforts, with new infections rising by dozens in recent days despite well-honed systems of mass testing and quarantine.

The flareup began after nine workers at the city’s airport were found to have been infected by the virus on July 20 during regular testing. The cluster quickly expanded to their close contacts, then to a handful of other locations, leading to a total of more than 150 infections as of Wednesday. It’s one of China’s biggest outbreaks since a winter wave concentrated in the country’s northeast saw more than 2,000 cases.

Officials have confirmed that the new outbreak is caused by the delta strain, which has been driving a resurgence in infections across the world. The highly-contagious variant is exposing the limit of the “Covid Zero” strategy targeted by governments from China to Australia, which had largely eliminated the virus only to see cases surge amid delta’s spread. The strain is increasingly breaking through China’s strict quarantine for overseas arrivals, and poses a challenge to the country’s repertoire of widespread testing and contact tracing.

Covid Delivers an Unsettling Reality Check to the World

Many of the people infected, including the Nanjing airport workers, have been fully vaccinated -- and only four have developed severe cases of the disease -- according to official data. The numbers signal that the immunity generated by Chinese vaccines, while enough to ward off critical illness and death, is still insufficient to prevent the spread of the variants.

Delta Circumvents

Chinese vaccines’ efficacy in preventing symptomatic Covid have ranged between 50 to 80% in studies, lower than the more than 90% recorded by potent mRNA vaccines developed by Pfizer Inc. and its German partner BioNTech SE, as well as Moderna Inc.

Countries including Thailand and United Arab Emirates that initially relied on Chinese vaccines have decided to offer booster shots to some fully-vaccinated people in the hope of providing better protection against the delta strain. Globally, the variant has already forced the U.S. to institute new mask mandates, delayed a reopening in Singapore and placed cities in Australia under quarantine.

China has also seen more frequent virus flareups since the beginning of this year as variants from other hotspots around the world exploit inadequate safeguards, like unthorough disinfection of high-risk workers who may have been exposed at airports and contaminated cargo centers.

The flares are putting pressure on officials to rethink China’s vaccination campaign to include booster shots. The drive -- the fastest in the world -- is just a month away from covering 75% of the country’s 1.4 billion people with two doses.

Sinovac Biotech Ltd., whose inactivated vaccine has formed the backbone of the inoculation campaign, said Wednesday that a third dose of its shot can increase antibody levels by three to five times, building a stronger case to give booster shots to workers at greater exposure risk of contracting imported infections.

Bloomberg also reported in April that China was expected to approve the mRNA vaccine developed by BioNTech. The shot could be used as a booster for people fully vaccinated with Chinese vaccines.

©2021 Bloomberg L.P.