An unexpected delay in shipments of Pfizer’s vaccine has complicated inoculation efforts in Mexico.

Mass vaccination of medical professionals across Mexico, including Sonora to Arizona’s south, started in mid-January. More than the recommended 21 days between doses of Pfizer’s vaccine have since lapsed, but Health Department Undersecretary Hugo Lopez-Gatell said a major shipment is expected soon.

In a recent letter to Mexican President Andrés Manuel López Obrador, the company said nearly 500,000 doses would arrive on Feb. 15, according to Gatell.

Second doses can be given up to 35 days after the first and still be effective, Gatell said.

More than 16,000 health professionals have received their first dose in Sonora, but none have received the second, according to a state Health Department spokesperson. Nationwide, roughly 13% have received the second shot, according to a recent presentation by federal health authorities.