SEOUL—North Korea has turned down roughly three million doses of Covid-19 vaccines developed by Chinese drugmaker Sinovac Biotech Ltd., instructing that they should instead be sent to harder-hit countries.

The shots were offered in recent weeks through the Covax initiative, a program financed mostly by Western governments to help lower-income countries obtain vaccines.

North Korea’s public health ministry declined the shipment, citing the limited global supply for Covid-19 vaccines and continuing virus surges elsewhere, according to a spokeswoman for Unicef, which helps deliver shots on behalf of Covax. North Korea requested the vaccines be “relocated to severely affected countries,” the spokeswoman said.

Kim Jong Un’s regime has reported zero confirmed Covid-19 cases to the World Health Organization. Its borders remain sealed, and state media has urged constant vigilance in the nation’s anti-epidemic campaign. Mr. Kim has called avoiding virus outbreaks a matter of national survival.

The impoverished country, which lacks the funds to purchase vaccines itself, has applied for assistance through Covax, though has yet to receive any doses. A planned Covax shipment earlier this year of about two million AstraZeneca PLC vaccines has faced delays.

North Korea has expressed some public doubt over the efficacy of Covid-19 vaccines. State media have reported incidents in the U.S. and Europe where vaccinated individuals have fallen sick. In May, the country’s main newspaper said, “The vaccine is not a panacea to all problems.”

Several countries that have used the Sinovac vaccine have expressed reservations over the level of protection offered by the shot. Some, including Thailand and Indonesia, have offered a booster shot of Western vaccines to healthcare workers who had received two Sinovac doses previously.

Clinical trials conducted earlier in the pandemic showed that Sinovac’s vaccine is around 51% effective against symptomatic disease, though little data exists on how well it can protect against new variants. The WHO has approved Sinovac, a private Chinese drugmaker, for emergency use.

Before declining the Sinovac vaccines, North Korea spurned the AstraZeneca shots over concerns of side effects, according to a July report by a South Korean think tank attached to Seoul’s spy agency.

Others have also tried to help the Kim regime. Russia’s foreign minister told reporters in July that Moscow had made multiple offers to supply its homegrown vaccines to Pyongyang. It isn’t known whether the Kim regime has accepted.

The U.S. and South Korea have recently discussed offering possible humanitarian assistance to North Korea, which could include Covid-19 relief.

Pyongyang’s claims of being Covid-free are doubted by health experts familiar with the country’s outdated infrastructure. Without widespread vaccination, North Korea is unlikely to be able to jump-start an economy that largely depends on cross-border trade with China.

North Korea remains on edge about pandemic slip-ups. In June, Mr. Kim, without specifying what had gone wrong, said the country’s Covid-19 situation had become grave and admonished top officials for lapses.

State media implores citizens to wear face masks in public. For inspection and quarantine-unit officials, state media said last week, “There can be neither any concession nor idleness in the campaign.”

Covid-19’s Delta variant is proliferating world-wide threatening unvaccinated populations and economic recovery. WSJ breaks down events in key countries to explain why Delta spreads faster than previously detected strains. Composite: Sharon Shi The Wall Street Journal Interactive Edition

Write to Timothy W. Martin at timothy.martin@wsj.com