One of the most important things that countries need to be doing to help understand and stop the spread of COVID-19 is testing. Here we explain why testing is important, what it involves, and how many tests countries are doing based on available data from official sources.

It’s important to remember that these numbers change very quickly, so they’re provisional and uncertain; but data about testing is extremely important, and at the moment there is no central database compiling them, so we are bringing together the best available numbers, after reviewing a large number of data sources from individual national reports, country by country.

Why is testing important?

Testing allows infected people to know that they are infected. This can help them receive the care they need; and it can help them take measures to reduce the probability of infecting others. People who don’t know they are infected might not stay at home and thereby risk infecting others.

Testing is also crucial for an appropriate response to the pandemic. It allows us to understand the spread of the disease and to take evidence-based measures to slow down the spread of the disease.¹

Unfortunately, the capacity for COVID-19 testing is still low in many countries around the world. For this reason we still do not have a good understanding of the spread of the pandemic.

How are COVID-19 tests done?

The most common tests for COVID-19 involve taking a swab from a patient’s nose and throat and checking them for the genetic footprint of the virus. They are called “PCR tests”. The first PCR tests for COVID-19 were developed very rapidly – within two weeks of the disease being identified – and they are now part of the World Health Organisation (WHO)’s recommended protocol for dealing with the disease.²

Here you can find an explainer video on how the tests for coronavirus disease work.

What information about test coverage do we currently have?

Ideally, we would like to have as much detail about COVID-19 tests as possible. We would like to know how many people in the world are being tested for COVID-19 every day, and in which countries. We’d like to know what the results of these tests are, and how the available tests are being used  – are countries testing only people in hospitals? People with symptoms?

Unfortunately, the WHO does not have a centralized database for COVID-19 testing data, so at the moment it is very hard to know.

That said, several countries do publish statistics on the total number of tests performed. But these reports are published across individual websites, statistical reports and press releases – often in multiple languages and updated with different periodicity.

Because a global overview was not available from a single source, at Our World in Data we are bringing together a large number of data sources from individual national reports, in order to provide a global picture with the best available estimates.

Below we show the most recent data as of 20 March 2020, 18.00 GMT. This requires a lot of careful work, but we will do our best to expand and update these estimates regularly.³

Total tests by country

These two charts show the most up-to-date estimates of testing levels that we’ve been able to find (as of 20 March 2020, 18.00 GMT). You’ll see that some countries’ data is older than others, meaning that they can’t be directly compared. For instance, the estimate for the province of Guangdong in China has not been updated since 24 February.

The first chart plots the total number of tests against the total number of confirmed cases. The more tests a country has done the higher it is; the more confirmed cases it has, the further it is to the right.

Naturally, since you can only get a positive test if you carry out a test in the first place, countries that have performed more tests will tend to have more confirmed cases. In other words: there is a positive correlation between tests performed and cases confirmed. That doesn’t necessarily mean that countries that have done more tests really have more cases, although in many cases they will.

However, the differences between countries can tell us important things. For instance, we can see that some countries have done more tests per confirmed case. The UK, for example, has done many more tests than other European countries with a similar number of confirmed cases.

The second chart plots the number of tests performed in each country. The most obvious point is that the data shows South Korea has done many more tests than other countries. That means we can expect that in South Korea the number of confirmed cases – positive tests – is closer to the real number of infected people than it is in other countries.

That makes it encouraging to see that the number of daily confirmed cases in South Korea has gone down. (Here you find our chart that shows the decline of confirmed new cases in South Korea.) South Korea was able to quickly manufacture and use a huge number of tests, showing that it is possible. Testing is crucial, so it is vital that other countries follow South Korea’s lead.

A note about the chart: There are now too many countries in our dataset to show them all at once, so not all countries with available data are shown by default. If you want to see a different country, you can select the option ‘add country’ to find it.

[NB. We provide two estimates for the US. The estimate labelled “US – CDC samples tested” is from the Centers for Disease Control and Prevention, and refers to the number of tests conducted, not the number of individuals tested. The estimates labeled “United States” correspond to estimates of people tested, according to data gathered by the COVID Tracking Project – these estimates are updated more frequently.]

Per capita tests by country

These two charts show the most up-to-date estimates of testing levels that we’ve been able to find (as of 20 March 2020, 18.00 GMT). You’ll see that some countries’ data is older than others, meaning that they can’t be directly compared. For instance, the estimate for the province of Guangdong in China has not been updated since 24 February.

The first chart takes the size of the population into account. It plots the total number of tests per million people, against the total number of confirmed cases, also per million people. The higher the dot representing a country is, the more tests per million people it has carried out; the further right it is, the more tests per million people have come back positive.

We see that countries with higher rates of confirmed cases tend to be also countries where a larger share of the total population has been tested. Again, since you can only get a positive test if you carry out a test in the first place, countries that have performed more tests will tend to have more confirmed cases.

But again there are important differences between countries. Vietnam, for example, shows a much higher testing rate than Indonesia, although at this point in time (20 March 2020) both have a similar number of confirmed cases per million people.

From this perspective, it is clear that the US is lagging behind. The number of tests per million people in the US is almost 10 times lower than in Canada, and about 20 times lower than in South Korea. The US has had big problems rolling out their testing strategy, although they seem to be slowly catching up.

The second chart plots the number of tests per million people, country by country. Not all countries with available data are shown by default – you can select the option ‘add country’ to see the available estimates for other countries.

The full documentation of our sources on testing data

We list estimates country by country, including exact dates and links to the underlying source, in a companion page here.

Some people require more than one test because of false-negative outcomes

The number of COVID-19 tests carried out will be similar to the number of people tested, but they won’t be quite the same, because some people may need to be tested multiple times. The reason for this is that there are “false-negative” test outcomes.^4,5,6,7 

A “false-negative” outcome is when someone is tested and found to be clear of the disease, but when tested again, are found to have it. The WHO’s guidelines for laboratory testing of COVID-19 say that negative results “do not rule out the possibility of COVID-19 virus infection.”⁸

This means that even in countries that have done lots of tests, the true number of COVID-19 cases is still uncertain, although of course more testing still means more certainty.

There haven’t been many studies into how common false negatives are, so it’s hard to know how big an impact they have on our understanding – but research is going on.⁹

Why might COVID-19 tests fail?

There are several reasons why someone infected with COVID-19 may produce a false-negative result when tested:^10,11

• They may be in the early stage of the disease with a viral load that is too low to be detected.
• They may have no major respiratory symptoms, so there could be little detectable virus in the patient’s throat and nose.
• There may have been a problem with sample collection, meaning there was very little sample to test.
• There may have been poor handling and shipping of samples and test materials.
• There may have been technical issues inherent in the test, e.g. virus mutation.

The WHO suggests that these issues should be taken into account and that for some people, tests should be carried out several times.¹²

You find data on confirmed deaths and cases, research on the disease’s symptoms and interactive visualizations of this data on our main page on coronavirus disease.