[This op-ed was originally published in the New York Daily News on Dec. 28, 2020.]

The arrival of vaccines against the coronavirus is the news we have been waiting for — the light at the end of the pandemic tunnel. But we can’t let that good news lead the city to abandon a careful approach to getting and keeping our schools open.

The hundreds of elementary and other schools recently reopened have shown very low levels of coronavirus infection — thanks in large part to the protective equipment and procedures the United Federation of Teachers, which I lead, insisted upon.

But the in-school testing that should provide an early warning system for rising infection rates is already strained, making it unlikely that the system could meet the challenge of testing a significant number of reopened middle or high schools. In addition, even a rigorous testing process in the schools will not be enough if the overall infection rate in the city skyrockets after the holidays.

New York can’t let its success in re-opening its schools be undermined by trying to open more schools beyond the city’s realistic testing capacity, or by keeping schools open in the face of a significant increase in community infections.

So I say to those hoping to open schools at all grade levels all across the city: We need to go slow.

The standards under which schools reopened this fall included the guaranteed presence of enough masks and face shields, improved ventilation, rigorous cleaning and classes small enough to ensure social distancing — and the most extensive virus testing regimen in the country.

The city’s current agreement with the state calls for 20% of the eligible school population to be tested every week. In terms of sheer numbers, the city has so far been successful in meeting its in-school targets, reaching tens of thousands of people in schools overall.

However, a number of problems have emerged.

In many schools, the overwhelming number of test subjects are adults, not students. Some schools — despite the pledge to reach every building — have not been tested at all.

More troubling is the fact that the city promised to return test results within 24 to 48 hours — yet thousands of results have not been posted within this time frame, some as many days after the test crews have visited.

As of Monday morning, Dec. 21, for example, nearly 1,000 test results from Monday, Dec. 14 had yet to be posted.

The whole point of regular, frequent testing is to identify individuals who show no symptoms yet carry the coronavirus and can spread it to other people in the building. Exposing students and staff to infected individuals for up to a week is completely unacceptable.

New York City’s virus testing capacity far exceeds that of any other jurisdiction, but it is already spread thin over more than 800 buildings and other program sites. Adding the now-closed middle and high school buildings to this mix would only result in less efficiency, slower tests and higher risk for all. That would be unwise.

Even with its current problems, the testing process has kept virus rates in the schools under control, thanks to its requirement that buildings be closed from 24 hours to two weeks if infections are discovered.

Under these guidelines, hundreds of schools across the five boroughs have closed temporarily. Even since Dec. 7, when pre-K, elementary and District 75 schools reopened, roughly 30% of those school buildings have been shut in response to a virus discovery.So I say to those hoping to open schools at all grade levels all across the city: We need to go slow.

Given this history, if community infection rates rise dramatically, infections will inevitably seep into schools, causing an even larger rolling schedule of shutdowns. At the same time, based on our conversations with outside medical experts, at some point in a period of high infections, schools could even become centers of virus spread.

We cannot afford to relax our vigilance. New York City — the only major urban center in the United States to reopen its public schools — has successfully done so because of its cautious approach and rigorous standards.

Remote instruction is far from perfect, but thanks to the efforts of thousands of dedicated teachers, it is getting better. To ensure that it can be as effective as possible, the Department of Education must do more to see that all students have working devices and all families have access to WiFi.

I appreciate how difficult it has been for many parents to deal with the challenges of remote learning, changing classroom schedules and temporary school closures. As a teacher, I also know that nothing can really replace the personal, face-to-face relationship between instructor and student.

In-person education is vital for our children’s future. But in the midst of this pandemic their safety, and that of their teachers, has to come first.

Mulgrew is president of the United Federation of Teachers.