The government said Saturday that it has signed a deal with Pfizer to import additional coronavirus vaccine doses for 20 million people from the US pharmaceutical giant.

With the latest deal, South Korea has secured enough Pfizer vaccine for 33 million people, including the doses it secured in December last year and February this year.

The deal will also raise the total number of COVID-19 vaccine doses the country has so far secured to an amount enough to inoculate 99 million people. This is 2.75 times the 36 million people needed to achieve herd immunity. Given the country’s total population of 52 million people, it is also nearly enough to vaccinate every Korean twice.

The Pfizer vaccine has a very good safety record. It uses messenger RNA technology, which seldom causes blood clots. Two shots are required, and clinical trials show that the product is 95 percent effective in preventing infection.

The government has authorized the use of the Pfizer vaccine for teens aged 16 and over. The authorities said they plan to supply the additional Pfizer vaccine doses starting in the third quarter of this year. But they did not disclose a concrete delivery schedule, citing the confidentiality of the contract.

US biotech company Moderna was to export enough of its vaccine to Korea for 20 million people starting in the second quarter of this year, but the delivery was postponed.

In December, the government said it had secured the Moderna vaccine after President Moon Jae-in personally spoke with the company’s CEO on the phone. Cheong Wa Dae even released a photo of Moon speaking with Moderna’s chief executive.

But acting Prime Minister and Minister of Finance and Economy Hong Nam-ki admitted April 20 that there was a hitch in the Moderna schedule.

People have come to realize that government announcements about vaccine deals can’t necessarily be taken at face value.

The same is true of the latest announcement. Quite a few people say it is hard to believe the government until they see the vaccine arrive with their own eyes.

Public concern is all the more intense considering the international vaccine supply situation.

The European Union is said to be on the verge of a deal with Pfizer to deliver 1.8 billion coronavirus vaccine doses from this year through 2023. Brussels says it is the world’s largest supply deal so far. Canada has reportedly reached an agreement with Pfizer for 35 million booster shots next year and 30 million in 2023. The country also has options for tens of millions more in future years should they be needed.

Considering these megadeals and the fierce international competition for Pfizer’s vaccine, Korea can hardly be sure of receiving the promised amount at the promised time. Big contracts with some countries cannot but have a negative impact on the supply situation for others.

Pfizer is reportedly expanding its production capacity -- but still, its contracted quantities exceed its capacity. There is no guarantee that problems will not arise with the supply of the vaccine or the operation of the production facilities. Furthermore, it is hard to conclude that any of these vaccines is risk-free.

Vaccines must be supplied at the right time if Korea is to achieve herd immunity as planned.

If an unexpected emergency happens, a rapid response will become difficult. The government ought to be ready for a sudden change in the supply situation. It would do well to work out a Plan B in case of setbacks with existing contracts.

The government should keep in mind that public confidence in its vaccine deals is low. After what happened with the Moderna vaccine, any similar delay will irreparably undermine public trust in government announcements.

Washington effectively controls the distribution of the Pfizer vaccine and other US vaccines. To secure a stable supply of Pfizer’s vaccine, the government in Seoul must also avoid offending the US with its divergent diplomatic approach.