How does coronavirus testing work and will we have a home test soon?

Because the symptoms of covid-19 are similar to those of other diseases, testing is the only way to know for sure if someone is infected with the coronavirus. 

HEALTH 25 March 2020

By Michael Le Page

A swab test is the best way to identify coronavirus infections

Roberto Pfeil/dpa

Because the symptoms of covid-19 are similar to those of other diseases, testing is the only way to know for sure if someone is infected with the coronavirus. Mass testing is therefore crucial to halting its spread. In the UK, a home test will apparently go on sale very soon.

How do you test for coronavirus infections?

At present, most tests are based on looking for genetic sequences specific to the covid-19 coronavirus. If these sequences are found in a sample, it must contain the virus.

What does testing involve?

Getting a sample to test involves pushing a swab – which resembles an extra-long cotton bud – deep inside the nose or to the back of the throat. The swab is then sent off to a lab.

What about testing blood or urine?

The virus is only detected in the blood, urine or faeces of roughly half of those who test positive based on nose or throat swabs, so blood, urine and stool tests aren’t reliable. If you are coughing up sputum, testing that can provide more accurate results than a nose or throat swab, according to a handbook summarising findings in China.

How long does it take to get a result?

Most labs use a method called the polymerase chain reaction (PCR), which takes several hours. It can take days for labs to run the tests and tell people their result. Several groups around the world, are developing faster genetic tests, typically based on a method called loop mediated isothermal amplification (LAMP), which takes less than half an hour. Handheld LAMP tests that could be used in homes and airports may start to become available within weeks.

Read more: How soon will we have a coronavirus vaccine? The race against covid-19

How accurate are the tests?

In theory, genetic tests should be extremely accurate if done properly. However, there have been reports from China of many false negatives and false positives. This may be because the swabbing wasn’t done correctly, or because overworked lab technicians were making mistakes. In addition, if people are tested very soon after becoming infected, they may not be shedding the virus yet.

Why is it so hard to get tested in most countries?

There are obvious practical issues with scaling up testing, from lack of trained personnel to equipment. But South Korea, which is now testing more than 20,000 people per day, has shown how fast it can be done. Many other countries didn’t start ramping up testing capacity until local case numbers began to soar and – unlike South Korea – haven’t made testing central to their strategy as advised by the World Health Organization.

I have heard some tests can be done in 10 or 15 minutes. How do they work?

Rapid tests, such as the one that may roll out in the UK soon, are usually based on detecting proteins rather than genetic sequences. These proteins can either be viral ones, called antigens, or the antibodies our bodies make to kill the virus. Antigen tests can directly detect the presence of the virus, but are less accurate than genetic tests.

Read more: We’re beginning to understand the biology of the covid-19 virus

What about antibody tests?

The downside of antibody tests is that they cannot detect infections in the first two weeks or so, when people are most contagious. However, our bodies keep making antibodies even after we have recovered from an infection, so testing people’s blood for antibodies against the coronavirus will reveal how many of us have been infected so far. This will help us calculate the infection fatality rate.

Can antibody tests distinguish between people who have recovered and those who are still infected?

Sometimes. People start producing so-called IgM antibodies against the coronavirus around 10 days after showing symptoms (perhaps 15 days after infection). After another two days, their bodies start making IgG antibodies, and gradually stop making IgM. Most people will recover fully as soon as IgG levels ramp up. Many rapid tests can detect both types of antibodies. If IgM antibodies are present in someone’s blood, they are likely to be still infected. If only IgG is present, they are recovering or fully recovered.

How accurate are these rapid tests?

We don’t know. Numerous companies are producing different tests that haven’t been independently checked yet. In general, testing for antigens or antibodies is less accurate than genetic testing, but the tests are easier and cheaper to manufacture. Rapid tests could play a valuable role, especially in poorer countries with little testing capacity. If they can reliably tell us when people have already been infected, it would allow key workers – especially in healthcare – to continue working without worrying about becoming infected or infecting their families.

Read more: https://www.newscientist.com/article/2238477-how-does-coronavirus-testing-work-and-will-we-have-a-home-test-soon/

Genetic Sequence vs Antibody tests - Will a home antibody test for covid-19 really be a game changer?

The UK has ordered 3.5 million antibody tests designed to reveal whether people have been infected with the new coronavirus. 

HEALTH | ANALYSIS 27 March 2020

By Michael Le Page

We might soon have a home test for covid-19

ED JONES/AFP via Getty Images

The UK has ordered 3.5 million antibody tests designed to reveal whether people have been infected with the new coronavirus. The UK’s prime minister, Boris Johnson, who today announced he himself has tested positive for the virus, has said these tests will be a “game changer”, but the reality is they might not have that much of an impact in the short term.

Almost all testing for the virus around the world is based on looking for its genetic sequence. But such tests require nose or throat swabs to be taken by trained personnel and sent to a specialised lab for analysis, and there is a global shortage of equipment. Genetic tests also detect only active infections.

Antibody tests, by contrast, detect the antibodies our bodies produce to kill the virus, which we keep producing even after the virus is eliminated. These tests can reveal who has been infected even after they have recovered. Handheld tests that require only a drop of blood can give results in 10 minutes, and can be mass produced quickly and cheaply.

Read more: How does coronavirus testing work and will we have a home test soon?

If we know someone has had the virus, they can potentially leave their home without risk of being re-infected, which would help countries get moving again. However, the accuracy of the tests has yet to be established. “The one thing that’s worse than no test is an inaccurate test,” Chris Whitty, the UK’s chief medical adviser, said on 25 March. Someone wrongly told they have already had covid-19 could go out and get infected.

How accurate do the tests need to be? “It’s very difficult to say,” says Emily Adams at the Liverpool School of Tropical Medicine in the UK, who is helping assess the tests developed by Mologic, one of the companies supplying the UK. Part of that process will be working out what accuracy is required for different uses, says Adams.

Ideally, we want to find out whether the thousands of health workers who are currently self-isolating because they or someone else in their home have symptoms that might be covid-19 can get back to work. Unfortunately, the antibody test may not help with this.

The antibody response to the coronavirus may be delayed compared with other infections. The tests can be used only 14 days or more after people develop symptoms, says Adams.

Read more: How soon will we have a coronavirus vaccine? The race against covid-19

This also means antibody testing will be of limited use for tracing the contacts of infected people – which many think is crucial for controlling the outbreak – because health authorities will be weeks behind.

Widespread antibody testing will also reveal whether large numbers of mild infections have gone unnoticed. It would be great news if this is the case, allowing many to return to work and meaning that the infection fatality rate is lower than thought. Unfortunately, places like South Korea that have been doing lots of genetic testing haven’t found vast numbers of mild cases.

On the plus side, many groups are working on faster genetic tests and on antigen tests that can detect the virus in, say, saliva. Testing widely for both active infections and past infections should be a highly effective combination.

Read more: https://www.newscientist.com/article/2238834-will-a-home-antibody-test-for-covid-19-really-be-a-game-changer/

Does a high viral load or infectious dose make covid-19 worse?

Does being exposed to more coronavirus particles mean you will develop a more severe illness? 

HEALTH 27 March 2020

By Linda Geddes

An illustration of coronavirus particles

MAURIZIO DE ANGELIS/SCIENCE PHOTO LIBRARY

Does being exposed to more coronavirus particles mean you will develop a more severe illness? Rumours circulating on social media suggest that hospital workers or their household members exposed to a higher “viral load” become sicker than the general population. But emerging research indicates the relationship between infection and covid-19 severity may be more complex – and differ from that of other respiratory illnesses.

The average number of viral particles needed to establish an infection is known as the infectious dose. We don’t know what this is for covid-19 yet, but given how rapidly the disease is spreading, it is likely to be relatively low – in the region of a few hundred or thousand particles, says Willem van Schaik at the University of Birmingham, UK.

Viral load, on the other hand, relates to the number of viral particles being carried by an infected individual and shed into their environment. “The viral load is a measure of how bright the fire is burning in an individual, whereas the infectious dose is the spark that gets that fire going,” says Edward Parker at the London School of Hygiene and Tropical Medicine.

Read more: You could be spreading the coronavirus without realising you’ve got it

If you have a high viral load, you are more likely to infect other people, because you may be shedding more virus particles. However, in the case of covid-19, it doesn’t necessarily follow that a higher viral load will lead to more severe symptoms.

For instance, health workers investigating the covid-19 outbreak in the Lombardy region of Italy looked at more than 5,000 infected people and found no difference in viral load between those with symptoms and those without. They reached this conclusion after tracing people who had been in contact with someone known to be infected with the coronavirus and testing them to see if they were also infected.

Similarly, when doctors at the Guangzhou Eighth People’s Hospital in China took repeated throat swabs from 94 covid-19 patients, starting on the day they became ill and finishing when they cleared the virus, they found no obvious difference in viral load between milder cases and those who developed more severe symptoms.

Although it is difficult to draw firm conclusions at this stage, such studies “may impact our assumptions about whether a high number of viral particles predisposes to a more serious disease”, says van Schaik.

Read more: We’re beginning to understand the biology of the covid-19 virus

However, a study of patients hospitalised with covid-19 in Nanchang, China, found a strong association between disease severity and the amount of virus present in the nose. “Those with more severe disease had a higher level of virus replication, although we have no evidence to relay the initial exposure dose to disease outcome,” says Leo Poon at Hong Kong University, who was involved in the study. “That rumour is still an open question to me.”

It is early days, but if the infectious dose doesn’t correlate with the severity of disease symptoms, this would mark covid-19 out as different from influenza, MERS and SARS.

For influenza, a higher infectious dose has been associated with worse symptoms. It has been tested by exposing volunteers to escalating doses of influenza virus in a controlled setting and carefully monitoring them over several weeks. This hasn’t been done with covid-19, and is unlikely to happen, given its severity.

Read more: How soon will we have a coronavirus vaccine? The race against covid-19

Animals infected with higher doses of the SARS and MERS coronaviruses also experienced worse outcomes, says van Schaik. “I think we just have to conclude that while this virus is related to SARS, there are also important differences that are currently poorly understood,” he says.

Even if the infectious dose isn’t related to disease severity, it still pays to try and minimise our exposure to the virus because this will reduce our chances of falling ill in the first place. “We want to be taking every precaution we can to prevent ourselves getting infected, which will also reduce our ability to pass the virus on to others,” says Parker. “Any measures we can take to avoid infection are worth taking.”

Read more: https://www.newscientist.com/article/2238819-does-a-high-viral-load-or-infectious-dose-make-covid-19-worse/