While vaccinations are a key to controlling the pandemic — reducing severe disease as well as transmission — they may still not be enough to stop it single-handedly. In a recently published mathematical model, scientists in Britain found that even if 75 percent to 95 percent of people in that country received a vaccine that’s 85 percent effective, the reproductive rate would remain greater than 1 — meaning the epidemic would continue to grow.

Of course, the rollout of the vaccines will take months to reach high levels of population coverage — and 1 in 4 Americans say they are unlikely to get vaccinated, according to polls. Add to this new variants, some which are more transmissible and deadly, and it’s clear we are at risk of a serious resurgence. Every tool for fighting the coronavirus remains essential, including masking (preferably better face coverings than cloth), improved ventilation in buildings, and more protections and support for vulnerable workers.

The case numbers bear this out. The United States is averaging over 58,000 new infections a day, and unfortunately, new cases seem to have plateaued, after dropping from a winter peak. Testing remains all-important and is needed to detect hot spots early, which explains why the Biden administration designated over $12 billion for testing in its recent stimulus bill. If we can drive case numbers down through all the means at our disposal, we may have a second shot at meaningfully deploying a fundamental epidemic control strategy the United States failed to get right the first time around: test, trace, isolate (or “TTI”). We will need that strategy to stamp out remaining transmission chains.

A number of countries — including New Zealand, South Korea, Vietnam and China — largely stamped out the pandemic through border closures and lockdowns followed by vigorous test, trace, isolate programs: Every time a new case was detected, the patient was placed in isolation (often outside the home); their contacts were identified; and those people in turn were tracked down, tested and isolated (if they tested positive) or quarantined (if they tested negative). These countries stopped their epidemics long before vaccines were introduced. In the United States, the failure to establish robust TTI programs means we have struggled with figuring out where, when, how and why transmission was occurring even when cases were at their lowest point last summer.

Contact tracing programs exist in many U.S. states, but they remain overwhelmed. They have been plagued at every step of the way — from limited testing to slow tracing to impractical, unsupported and unenforced isolation. According to Test and Trace, an initiative to assess state-level contact tracing capabilities based on test positivity rate, speed of test results and the size of the contact tracer workforce, no U.S. state is ready to effectively implement TTI.

The World Health Organization says a key performance indicator for successful tracing is the quarantine of 80 percent of contacts within three days of an index case’s symptoms — a feat few countries have been able to achieve (and the United States has not come close).

To stamp out pockets of persistent transmission after we push down case numbers, we need a smarter and more sophisticated TTI infrastructure. One weapon is rapid antigen-based tests. They are cheap and can be used regularly and frequently to identify highly infectious cases — potential superspreaders. Unfortunately, antigen tests can have a fairly high false positive rate, which is why confirmatory testing with PCR (or, alternatively, repeat antigen testing) is important, too. (False positive test results have led to unnecessary tracing, which wastes the resources of health departments.)

We can use tests in creative ways that we didn’t try at scale before. Pooling PCR tests, especially when transmission levels are low, is a way to run far more tests in a single session. (Samples from multiple people would be combined; only in the case of a positive test would people be tested individually).

Tracing techniques include both the familiar “prospective” kind (identifying whom an index case went on to infect) as well as the “retrospective” variety (figuring out who infected an index case); modeling has suggested that retrospective tracing may actually be even more important than traditional forward tracing. One of the most challenging parts of tracing the coronavirus is asymptomatic spread, but digital tracing apps can fundamentally improve how well we do this, if we have enough public uptake.

With digital tracing, when an asymptomatic case tests positive, anyone who had recently been physically near them immediately gets notified via mobile devices. The experiences of South Korea and other East Asian countries show the capabilities of such systems. These tools were met with hesitancy in the United States earlier in the pandemic because of data privacy concerns, but innovations have made them more secure.

Lastly, isolation must meet people’s needs. People understandably don’t want to leave their families. Measures to improve the safety of home-based isolation could include supplying portable air cleaners and advising people to open their windows — and providing family members with medical-grade masks. Moreover, people need to be guaranteed financial support while they are out of work and protection from losing their jobs; they should not have to choose between their health and their livelihood. For those without safe living conditions, isolation spaces outside of the home should be offered, such as in repurposed hotel rooms or in field hospitals (like one created at the Boston Convention Center last year).

Some may recognize the value of test-trace-isolate systems but question whether strengthening them is worth the investment at this point in the pandemic. Why not focus all our efforts on vaccinating people as quickly as possible? But the two approaches are complementary. Moreover, TTI infrastructure will be critical not only now, but for the future. We will probably have to revaccinate our populations with booster shots for covid-19, and TTI will remain critical for other infectious-disease outbreaks if and when they occur.

We still have too many covid-19 cases to make testing, tracing and isolating work — but that may change soon. And better TTI systems could be one key to finally ending the epidemic.