Guangdong, Zhejiang and Henan provinces will start administering third doses of vaccine to residents this month to strengthen their immunity to COVID-19 as sporadic outbreaks are reported in some parts of China.

Those targeted for a third dose are people aged 18 to 59 who received two doses of coronavirus vaccine at least six months ago, authorities said.

The city health commission in Shanwei, Guangdong province, said on Thursday that priority for the booster shot will be given to front-line personnel at medical institutions and workers in government departments, law enforcement, public services, customs, border ports, harbors, the cold-chain industry and those who want to study abroad.

Shanwei, in the east of the province, is the first city in Guangdong to roll out booster shots. Many of its residents received their second dose more than six months ago.

Zhejiang and Henan will also begin administering third doses of vaccine to residents this month, according to local health authorities.

The health commission in Shanwei said research has shown that a person's antibody level will be increased more than tenfold if a third dose of vaccine is given more than six months after the second, and that immunity will be maintained at a relatively high level for another six months.

Government departments are sparing no effort to prevent and control the spread of locally transmitted cases in East China's Fujian province and Northeast China's Heilongjiang province.

In Harbin, Heilongjiang's provincial capital, one residential community in Nangang district and a residential community, three residential buildings and a community health center in Bayan county were declared medium-risk areas for COVID-19 from Thursday, the city's epidemic prevention and control headquarters said.

Harbin reported eight new locally transmitted confirmed cases of COVID-19 on Wednesday, bringing the total to 11 since the first case in the latest outbreak was reported on Monday.

Fujian reported 20 local cases on Wednesday, including 17 in Xiamen and three in Putian, the provincial health commission said on Thursday.

From Sept 10 to Monday, Fujian reported 438 local confirmed cases, with all of them being hospitalized and no deaths recorded.