Health authorities are scrambling to adjust their coronavirus inoculation schedule after several delays in the delivery of vaccines.

On Tuesday, the Korea Disease Control and Prevention Agency said it could extend the interval between jabs of AstraZeneca vaccines to ensure that more people can get their first shot in time. Earlier this month, health authorities set the interval between the first and second AstraZeneca injection to eight weeks, only to change it to 10 weeks shortly afterwards as supply problems emerged.

Now, they want to expand the interval even further to 12 weeks due to a shortage expected in the second quarter.

A shipment due Wednesday of AstraZeneca vaccines that the government was promised through the WHO’s COVAX Facility has been delayed for three weeks, while the shipment volume has also been slashed.

“In spite of the unstable supply of vaccines, we will proceed with scheduled inoculations to achieve herd immunity by November,” the KDCA said Tuesday. “We will increase the rate of inoculation by maximizing the use of AstraZeneca vaccines that have been stored for second injections.”

AstraZeneca doses can be administered up to 12 weeks apart, but Pfizer vaccines must be administered at an interval of three weeks for maximum efficacy.

There are hopes that Novavax vaccines manufactured in Korea could ease the supply shortage. SK Bioscience, which has the manufacturing license, is reportedly “almost ready” to start production at its factory in Andong.

But there is a shortage of raw ingredients, and global authorization is still pending and is not expected until June in the U.S. and U.K. That means the next three months look bleak.

Korea is falling further and further behind most advanced countries in the race for vaccination.

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