Covid-19 tests still matter even with vaccines available

The pace of vaccination for Covid-19 in the United States is accelerating, with shots topping 2 million per day. At the same time, deaths, hospitalizations, and new cases are dropping from their winter peak.

But another essential line of offense against the coronavirus is going in the wrong direction: Testing for Covid-19 has been declining from a high of 2 million tests daily in January, to about half of that this month.

Researchers alarmed by this drop-off say it leaves the US vulnerable to missing large, new Covid-19 outbreaks, which could require more shutdowns to control. And without adequate testing, new, more harmful variants of the virus could run rampant undetected.

The critical role of testing is a lesson that the US has struggled to learn throughout the pandemic. At the beginning, before health officials understood much about the virus, it was clear that to stop it, they needed to find it. Some argued in those early days that we should have been doing upward of 35 million tests per day if we wanted to avoid shutting down schools, businesses, and public venues, and the punishing economic costs of doing so. If we missed that window, they said, the country would end up with millions sickened and hundreds of thousands dead.

Of course, the US ended up with both a massive death toll and an economic crisis. And the ambition of testing for Covid-19 on this scale was never realized. The US never developed a coherent national testing strategy.

But it’s not too late.

In fact, in the current moment, testing for Covid-19 may be more important than ever. “I think the testing pillar of the pandemic response is still as vital as it’s ever been,” said Joseph Petrosino, chair of molecular virology and microbiology at the Baylor College of Medicine. “Not only do we need to test, we need to start identifying which variants of the virus are spreading in a given area.”

Several of the new variants of the virus are more transmissible, may lead to more dangerous outcomes, and are starting to dominate new infections, threatening to roll back some of the progress in corralling the pandemic. So testing is crucial to keeping tabs on these mutant strains.

And while new Covid-19 cases seem to be declining, part of that may be due to a drop in the testing rate. The SARS-CoV-2 virus that causes Covid-19 is still wreaking havoc, killing hundreds every day.

Testing, then, remains one of the most important tools for keeping track of the virus and could serve as an early warning sign of changes to the virus itself.

It’s also a means to get back to regular life as soon as possible. At current rates, it will likely take months using vaccines to reach herd immunity, the point where the virus cannot spread easily in a population because a sufficient number of people are immune. Deploying a more thorough testing strategy could still be a means to relaxing pandemic restrictions on a faster timeline.

Why Covid-19 testing is still essential even when we have vaccines

Testing for Covid-19 is a versatile tool, and when deployed as part of a larger public health strategy, it has proven effective at containing the virus. Places like Taiwan, Singapore, South Korea, Australia, and New Zealand, which all rapidly rolled out a widespread testing strategy early on in the pandemic, were able to relax restrictions on movement and allow life to return to much closer to normal, even before distribution of Covid-19 vaccines began. In the US, sports leagues like the NFL and the NBA managed to continue games in part due to rigorous and repeated testing.

But to use tests effectively, one must first consider why we want to use them. “Any conversation about testing has got to start by saying there’s these at least three conceptually very different use cases,” said Paul Romer, an economist at New York University who has been studying Covid-19 testing throughout the pandemic. “When you talk about ‘we need more testing,’ the first question you’ve got to ask is ‘more testing for what purpose?’”

One purpose is clinical diagnostics, where the results are used to inform health care decisions about the person getting tested, like whether they should be isolated from others, whether they should commence treatment, and whether their close contacts should quarantine.

The next category is disease surveillance. Here, the results of Covid-19 tests are not necessarily revealed to individuals but are instead aggregated across a large population to keep track of how the virus is moving. This is used to help inform public health strategies.

The third category is screening. That’s where large groups of people who may not necessarily have a high likelihood of exposure are still tested regularly. Screening for Covid-19 can help people get back to work or get kids back to school.

The problem is that the US has been very reactive with how it deploys Covid-19 tests, using them mostly to confirm suspected cases rather than seeking out and filtering unseen infections. “We have generally as a nation overinvested in the use of tests in clinical care and underinvested in both surveillance and screening,” Romer said. So shifting toward a more proactive strategy in looking for the virus in broader groups of people would help contain new pockets of infection and permit more public spaces to reopen, even without widespread vaccines.

Depending on the use, different types of tests are ideal. The gold standard for diagnosing Covid-19 remains RT-PCR tests. They are highly specific and sensitive, but they’re expensive, tedious, and time-consuming to run. In recent months, rapid antigen tests have come to the fore. They’re much cheaper and faster — many generating results within minutes — but they make some trade-offs in accuracy. However, when conducted repeatedly, their detection capabilities improve further. That makes them ideal for proactively screening people on a regular basis for Covid-19.

“I think it’s really too bad that we haven’t made better use of rapid tests,” said Emily Gurley, an associate scientist studying epidemiology at the Johns Hopkins Bloomberg School of Public Health. “They are very imperfect but they do give us some important information.”

One of the reasons these tests have been slow to get off the ground in the US is that the Food and Drug Administration doesn’t authorize tests just for screening and is mainly interested in tests that are used for clinical diagnostics, according to Romer. That holds tests to a much higher bar than may be needed for quick, cheap, and frequent screening purposes.

However, if the US were to shift toward a more proactive approach to testing using fast and cheap tests, it would benefit the vaccination campaign as well. With demand for vaccines still far outpacing supply, Covid-19 testing can help optimize where vaccines are allocated. For example, identifying hot spots and then vaccinating people around the hot spot, an approach known as ring vaccination, is a proven strategy for containing disease outbreaks. It’s been used to contain smallpox and Ebola. And the test, trace, and isolate strategy can still work to keep transmission low. Alongside vaccinations, these actions would be far more effective at containing Covid-19 than either approach alone.

Covid-19 testing is moving in the wrong direction — at exactly the wrong time

Even though many of the Covid-19 metrics seem to be trending the right way, they don’t tell the whole story and could even mislead us about the current trajectory of the pandemic.

For instance, one key metric that health officials pay attention to is test positivity, the fraction of tests that yield a positive result. It’s a shorthand for how much transmission is occurring. The World Health Organization has set a benchmark of 5 percent positivity or lower for two weeks before relaxing Covid-19 restrictions. The US test positivity rate has been around 4.5 percent for several weeks.

“The thing that makes me worried, even though test positivity is so relatively low compared to where it has been, is the fact that this decline in testing seems to start before we saw the decline in cases, which tells me that something else is happening to make testing scale back,” said Jennifer Nuzzo, an epidemiologist and a senior scholar at the Johns Hopkins Center for Health Security.

How tests are reported can confound these numbers even more. The test positivity rate can change dramatically depending on whether clinical diagnostics, screening, and surveillance are included in the total. So including a batch of daily screening tests, for example, while lowering the number of diagnostic tests could make the positivity rate appear lower than it actually is.

And the overall numbers can obscure some of the finer details about the spread of the virus, like the locations or communities that are driving new cases.

But despite the well-established need for widespread Covid-19 testing, the number of tests for Covid-19 is declining.

There are a number of reasons this may have happened. As the pandemic progressed, the number of survivors of Covid-19 grew. Now almost 30 million people in the US have been infected at some point. People who have survived the virus may not bother to get tested again. There is also some pandemic fatigue at play, so people who were potentially exposed might not bother getting tested and those with symptoms may not seek out treatment and stay at home.

With vaccines rolling out, some health departments have also been forced to reallocate resources away from testing and toward vaccinations. And as a greater share of the population gets vaccinated or survives the disease, the demand for testing is likely going to wane further. That could have important consequences for the long-term effort to contain Covid-19 (more on that below).

“There’s no one explanation for it, but certainly those things are going to contribute,” said Gurley, the Johns Hopkins epidemiologist.

And it’s not just the declining number of tests that’s concerning, but how quickly they produce answers. If tests take too long to yield results, the information they provide can become moot. Many parts of the country continue to experience long wait times for Covid-19 test results.

“Even a day’s delay can hamper your ability to stop transmission going forward,” Gurley said.

The Biden administration announced last month that they would invest $650 million to expand Covid-19 testing across the US aimed at schools and underserved groups, a step toward the White House’s national testing strategy. Biden campaigned on doubling the number of drive-through testing sites and creating a pandemic testing board to manufacture and distribute tens of millions of tests.

But while health experts say that testing has to be integrated as part of a larger, multi-step pandemic response, it’s been hard to maintain public attention on more than one thing at a time — masks, social distancing, vaccinations, and so on.

“Most public health experts will tell you that it’s not about one intervention, it’s about layering on interventions,” said Bruce Y. Lee, executive director of public health computational and operations research at the City University of New York. “The effectiveness of vaccines or effectiveness of masks, all those things really depend on how well you contain the virus, and all that depends on knowledge of where the virus is spreading, and all that depends on testing.”

And testing is shaping up to be the first line of defense against new variants of the virus. Because some variants are more transmissible, they’re changing the expected patterns of the spread of the virus. Covid-19 caused by the new variants may even be presenting with different symptoms in some cases. Without adequate surveillance, these variants may be far more widespread than realized.

The problem is that most Covid-19 tests can’t distinguish between variants. The way scientists identify new mutations and variants of the virus is by genetic sequencing, a process that currently takes place outside of most routine testing. Here, too, the US is falling behind. So a combination of more surveillance tests and more genetic sequencing of SARS-CoV-2 would help ensure the US isn’t caught off guard by any more changes in the virus.

We are going to need testing for the foreseeable future, even after the Covid-19 crisis passes

The fact that three highly effective Covid-19 vaccines are being distributed around the country is excellent news and represents the clearest route out of the pandemic. But there are still some unknowns about how well vaccines work, especially with new variants in play.

There is increasing evidence that the vaccines are excellent at preventing not only deaths and severe disease but also transmission of the virus among people. However, it’s not yet clear how long these protections will last. (Serological tests for antibodies could be one way to see whether protection holds up, although the concentration and effectiveness of antibodies are not the only component of immunity.)

Several SARS-CoV-2 variants also seem to be better able to evade the immune protection provided by a previous infection or from some vaccines. So continuing to test for Covid-19, even among people who are vaccinated, will be critical. Otherwise, the likelihood of more dangerous or more evasive variants of the virus spreading undetected will grow.

“If somebody gets sick who has been vaccinated, that’s actually more reason to get tested, because you want to identify whether they have a variant that escaped a vaccine,” Petrosino, the Baylor microbiologist, said.

Getting ahead of these variants requires a two-step testing process that would first determine whether a person is infected and then figure out which variant is infecting them, which adds cost and complexity to the testing process. But building that capacity would help ensure fewer surprises.

All the while, parts of the US are already letting up on some of their control measures like mask mandates, which some health officials say is premature and could end up prolonging the spread of Covid-19, especially without a strong testing strategy to fall back on.

And while it may take months for everyone in the US who wants a vaccine to get one, it could be years before the same is true for the rest of the world. That means pockets of infection may remain where the virus can continue spreading and mutating, eventually posing a fresh threat to the US.

“Once we reach herd immunity in the United States, that’s wonderful, but it doesn’t really help if we don’t focus on vaccinations across the world,” Petrosino said. During that time, testing will remain the most important tool in detecting a reemergence of the disease.

“There is a non-trivial chance that we’re going to have another outbreak,” Romer said. “And we’re going to have another emergency where people are going to say we’re going to have to lock down again.”

It also isn’t too soon to start thinking about the next disease outbreak or pandemic. Many of the lessons learned with Covid-19 can apply to other infections, particularly the need for vigilance. When it comes to infectious disease, no one can let their guard down, and a robust testing system is the best way to keep eyes on the adversary.