LONDON, ONT. -- Mass vaccination clinics in London and Middlesex County could stop administering first-doses of vaccine if the supply of Pfizer and Moderna shots doesn’t increase significantly.

The Chief Operating Officer at the London Health Sciences Centre is warning that appointments scheduled months ago for second doses will have to take priority.

Neil Johnson tweeting his concern, “Now we need to double our current supply to keep at (the) current pace as delayed second doses are coming due soon! If no new supply, first doses will stop.”

The Pfizer, Moderna, and AstraZeneca vaccines all require two shots before offering their maximum protection from COVID-19.

“We are starting to see some of the second doses that were required from earlier in the (vaccination) campaign come into play over the next few weeks,” Medical Officer of Health Dr. Chris Mackie told a media briefing on Monday.

On Dec. 23, the first COVID-19 vaccine dose was administered at the Western Fair Agriplex.

Vaccination accelerated over the winter, leading to a wave of partially-vaccinated people requiring a second shot soon.

“That will now be a factor in the month of May for sure,” explains Dr. Mackie. “The upside though, is we do expect a lot more Pfizer to come on board in May and June.”

Last week, Middlesex-London learned it will have about 25 percent of its Pfizer doses redirected to hotspots in Toronto and the GTA.

Dr. Mackie is still anticipating, at this point, a surge of vaccine in May and June to meet the demand for both first and second doses.

“So if the national procurement goes as expected, then the second doses required will be dwarfed by the new incoming doses. We would be able to proceed at pace for vaccinating people with first doses,” adds Mackie.

As of April 19, more than 125,000 doses of COVID-19 vaccine have been administered in London and Middlesex County.