Zhuhai in South China's Guangdong Province had vaccinated over 1.4 million residents by Tuesday, becoming the first Chinese city to have realized a vaccination rate of over 80 percent of people between the ages of 18 and 59.

Experts said it laid a solid foundation for the establishment of an immune barrier, while reminding the public that the vaccination drive should not be slowed down until over 90 percent of the population are given jabs.

The mainland city, which connects Hong Kong and Macao, launched its mass vaccination drive on March 21, according to local media reports.

The number of inoculation venues at the city has expanded from 64 to 450, and capacity has increased from 5,700 doses per day to over 100,000 per day. Nearly 3,000 healthcare workers have been dispatched to the sites in the past month.

Zhuhai also purchased a batch of 100,000 doses of vaccines in addition to distributions from Guangdong provincial health departments, and the batch was delivered to inoculation sites as quickly as within four hours.

Experts said that Zhuhai's practice has laid a solid foundation for building an immune barrier against virus imports from overseas, and also provides experience for rapidly building herd immunity in port cities with frequent population movements.

However, the pace should not be relaxed as the vaccination rate is still not high enough to ensure herd immunity. "Since vaccine efficacy differs from person to person, over 90 percent of the population need to receive the doses to guarantee the immune barrier," Yang Zhanqiu, a deputy director of the pathogen biology department at Wuhan University, told the Global Times on Thursday.

After the neighboring Hong Kong Special Administrative Region was recently hit by a wave of COVID-19 imports from India, with 53 passengers on a flight from New Delhi to Hong Kong on April 4 testing positive for coronavirus, some have expressed worries that the mutated virus arriving from the Indian flight might affect China.

Yang said such worries are unnecessary, as the fundamental characters of the mutated strains have not changed despite minor adjustments, and China's vaccines, which target the core characters of the virus, have proven effective in fending off variants.