Why a negative Covid-19 test before Thanksgiving isn’t an all clear

More frequent Covid-19 testing is one of the keys to helping end the pandemic. But as individuals, we cannot rely solely on testing to protect ourselves and others.

Take this recent example: public health officials in New Zealand reported on a group of cases which probably spread on a long-haul flight. Still, the suspected index cases – people who likely spread the virus to others – tested negative for the virus 72 hours before boarding their plane in Switzerland and believed they weren’t infected.

This shows: A negative test is unclear in terms of the ability to safely interact with others without masks and other precautions. “Negative tests are not like a passport for people to go out and do whatever they want,” as Muge Cevik, virologist and doctor at the University of St. Andrews told me earlier this fall.

As people (recklessly) consider traveling this Thanksgiving week, they need to understand that testing negative doesn’t mean it’s safe to be in close contact with other people. This does not mean that it is safe to remove the masks.

Scientists do not yet know exactly when a person infected with the coronavirus will start testing positive for the virus. There are situations where a person could test negative, be infected, and also be contagious. It is also possible – since the virus multiplies exponentially in the body very quickly – that someone could test negative in the morning (and not be contagious), but in the afternoon test positive (and be very contagious).

Confusing? Yes it is. But the bottom line is that the Covid-19 diagnostic tests (both the slowest and most common viral genetic test – called RT-PCR – and the faster viral protein test, called the antigen test) are more accurate when used on people with symptoms. .

“One of the huge data gaps right now is, what is the likelihood of being tested positive before having symptoms?” Benny Borremans, a disease ecologist at UCLA, said in October. At the moment, scientists are just not sure.

Why tests are less accurate before symptoms start

There are several reasons why scientists don’t know when people will start testing positive for SARS-CoV-2. To understand why, and to make it less confusing, it helps to think about all the things that need to happen for a Covid-19 test to come back positive.

First, the virus needs time to establish itself in a person’s body. This is called the incubation period and it can take up to two weeks. On average, this happens in about five or six days. During the incubation period, a person may not test positive because there is not enough virus in their body to be detected in a test.

“The virus particles, day after day, will multiply,” Cevik says. “The virus must reach a threshold for PCR [i.e. viral genetic] tests to pick it up. PCR is the most common Covid-19 diagnostic test because it requires a lower threshold of the virus to be positive; rapid antigenic tests would require a higher level of virus to register a positive test.

The positive test must coincide with being contagious. But not always.

Usually, a person can start to be contagious to the virus about two days before they start showing symptoms, in what is called the presymptomatic phase.

And, generally – but not always – scientists expect that if a person is contagious, they will test positive. After all, if they cough up enough virus to make another person sick, they are probably coughing up enough virus that a diagnostic test can be detected.

But when exactly a person goes from a negative, non-infectious test to a positive, infectious test is difficult to predict.

“If everything is working as it should, the test should be positive if you are infectious at the time of the test, because then there must be a virus,” says Justin Lessler, an epidemiologist at Johns Hopkins University. “However, you can easily test negative and then become contagious a day or even hours after the test.” Unless you test every hour, it’s impossible to have an accurate view of when the infectious period really begins. (Also possible, but probably rarer: a person tests positive before they start to be contagious.)

Even if a person is contagious, they may not test positive. It can be boiled down to the origin of the sample to be tested.

In general, “we consider the gold standard to be the nasopharyngeal swab,” says Bobbi Pritt, director of clinical microbiology at the Mayo Clinic. “It’s the deep nasal swab that goes up to the back of your nose. While other specimens – like a throat swab or just the very outer edge of your nose, like just inside your nostril – won’t contain as much virus.

At the onset of infection, a person who incubates the virus is expected to test negative. Over the summer, Johns Hopkins researchers – including Lessler – published an article estimate the likelihood of a false negative test in the first few days after being exposed to the virus. On the first day, they found the probability of a false negative close to 100%. No test will detect the virus so soon. In the first four days, this rate drops to 67% on the fourth day on average, but with a very wide error range. The day people first reported symptoms, there was still a significant false negative rate, at 38%.

What does it all add up to? “What we’re saying is don’t test anyone within four days of exposure,” Cevik says. It won’t tell you much about the person’s status. Or if a person is tested during this time, they should be tested again a few days later.

“Usually five to eight days after exposure is the best time to test,” says Cevik. “Or the third day after symptoms appear.” This is when genetic RT-PCR tests are most likely to show a true positive.

Because nothing about Covid-19 can’t be simple, here’s another thing to consider: Antigen tests that produce rapid results have a shorter window in which you would expect a person to test positive. .

They are also slightly less precise, but this limitation can be overcome if they are used repeatedly. If used correctly, these tests will report a positive result in the window when a person is most likely to be contagious. And with repeated use, scientists are hoping that these rapid tests could help prevent outbreaks from growing out of control.

A negative test without symptoms may not mean much. Keep your mask on.

Here’s the gist: “We don’t know when we’ll test for positive symptom appearance before PCR or antigen testing,” writes epidemiologist A. Marm Kilpatrick in an email. He points the finger some papers that try to quantify the likelihood of testing positive when asymptomatic, but it is difficult to draw conclusions and recommendations.

This is because the incubation period – the time it takes a person after the initial exposure to become infectious – can vary widely from person to person. (This can happen in four or five days, or up to two weeks.) “If someone has a long incubation period, our [work] suggests that their infectivity increases later and therefore there would be a longer period in which they would test negative.

If you have symptoms, you will likely be positive the day you start to feel sick, but that is not guaranteed. The first few days after you start to feel sick, you are very likely to test positive.

We may learn more in the coming months about screening asymptomatic and presymptomatic people, with studies following people after they’ve been exposed to the virus and repeated testing for a few weeks to determine the likelihood of testing positive. before the onset of symptoms. “We have a lot of data from the onset of symptoms, but we don’t have data in terms of pre-symptoms,” Cevik says.

This is why testing is not a substitute for other mitigating measures for Covid-19, such as quarantining those exposed to the virus, wearing masks and social distancing. Please remember this.