The latest cluster of local COVID-19 infections in Nanjing, capital city of Jiangsu province, has grown to nearly 180 people and spread to Anhui, Guangdong, Hunan, Liaoning, and Sichuan provinces, Beijing and the Inner Mongolia autonomous region.

Considering the greater contagiousness of the Delta variant, which has been found responsible for the Nanjing cluster, the fact that the new outbreak started at an international airport, many of the infected have complicated travel histories and, in particular, that we are at the start of the summer travel season, the situation may get worse as time goes by.

Several of those infected caught the virus as they transited at Nanjing airport and unknowingly brought it to other places. A number of others got infected while watching a show at a tourist attraction in Zhangjiajie, Hunan. Almost all of them traveled by public transport. The chains of infection must be longer and a lot more complex than is so far known.

In one latest case in Hulun Buir, Inner Mongolia, three people who were infected took a flight from Penglai, Shandong province, to Tianjin municipality, went to a relative's home by taxi, traveled to the local railway station by subway, from where they took a train to Beijing South Railway Station, then went to the Beijing Railway Station by subway to change for the train home.

Local authorities are making all-out efforts to find those who may have had close contact with the already identified victims. But it will prove a challenging mission, as nine days have passed since the Nanjing cases were confirmed.

As mass inoculation of COVID-19 vaccines unfolds and more and more people get their jabs, many people have begun to relax their vigilance against potential risks. The new infections may or may not suffice to make many rethink their summer vacation plans. But they should serve as a timely warning for everyone that they must remain on their guard.

Scientific data show that although the vaccines do not prevent people being infected, they do lessen the severity of the symptoms. Medical and epidemic control authorities must redouble their endeavors to raise public awareness of the need for people to be vaccinated, so as to make sure the country's mass vaccination program doesn't lose steam.

Meanwhile, the latest infections show the importance of people adhering to the fundamental protocol of personal protection that has served the country so well from the very beginning, such as mask wearing and social distancing.

At the end of the day, the virus may become less lethal, and more seasonal flu-like. But until then, vigilance remains essential.