The symptoms of COVID-19 - fever, cough, difficulty breathing and muscle pain - can resemble those of many other diseases, such as influenza, making diagnostic tests therefore essential for identifying people who actually have COVID-19. In addition to this, these tests can also help determine who has recovered from COVID-19, as well as improve our understanding of how the virus spreads and help monitor the effectiveness of control measures. So what kinds of tests are there and what challenges do they present?
Testing for the virus itself vs antibodies
Some test for the virus itself, by looking for the RNA (the genetic blueprint) of the SARS-CoV-2 virus that causes COVID-19. When carried out properly, a result that the virus has been detected is extremely reliable. However, these tests are not very helpful for determining whether someone has recovered from the virus, and moreover can potentially miss the virus if it is present in extremely low levels in a patient’s body.
Other tests look for antibodies to the virus – evidence that the body has produced an immune response to it. It takes time for such antibodies to be created, so antibody tests are not much use in confirming if someone has COVID-19 in the first few days of infection. However, in contrast to the RNA tests, they can be extremely useful in determining whether someone has previously been infected with the new coronavirus, but no longer has the virus present.
A complicating factor, however, is that different people can have different antibody responses to COVID-19. For example, individuals with severe disease seem to develop higher antibody levels than individuals with mild or asymptomatic disease. As a result, a test for antibodies developed using blood samples from individuals with severe COVID-19 may not work as well in detecting antibodies in people with a mild or asymptomatic version of the disease, where there are far fewer antibodies to detect.
How long does COVID-19 immunity last and what does it mean?
Whether or not COVID-19 antibodies confer long-lasting and complete immunity is not yet entirely clear. Infection with a contagious disease doesn’t necessarily lead to complete protection forever. With some diseases, such as measles, recovery essentially produces complete immunity to future infection for life. Recovery from other infections can be different. For example, respiratory syncytial virus - which can cause severe viral pneumonia in young children but usually only causes mild, cold-like symptoms - is so common that most infants have had it by age two. However, infection triggers only partial immunity which protects against severe disease in the future, but does not protect against re-infection.
With coronaviruses, some forms prompt complete immunity while others cause only partial immunity, and it is not yet clear where COVID-19 falls along that spectrum. If recovery from this new coronavirus only produces partial immunity, it would mean that someone with COVID-19 antibodies could still be infected by the SARS-CoV-2 virus, and while they themselves may be protected against severe disease, they could still potentially infect others. In other words, someone with partial immunity to COVID-19 could get re-infected and only have cold-like symptoms. But if they spread it to someone without any COVID-19 immunity, that person could get severe disease such as pneumonia.
The possibility that COVID-19 infection only induces partial immunity is a major reason why it is not yet clear if “immunity passports” would work. These are meant to indicate if a person has recovered from COVID-19, cannot infect others and can go about their activities without restriction.
The global scale-up of COVID-19 diagnostic testing has been rapid, literally going from zero to over a million samples tested a day in a matter of months. This scale-up in testing has been valuable both for guiding patient care and informing public health decision-making, including on implementing physical distancing measures.
However, there have been numerous difficulties, especially in low- and middle-income countries. Despite the rapid scale-up in testing, demand continues to outstrip supply, and distribution of the available supply has been far from equitable across countries. Ensuring adequate test quality and reliability has been a challenge, particularly given the many manufacturers who have entered this market, although the World Health Organization, the Foundation for Innovative New Diagnostics, and other review groups have played a key role in evaluating COVID-19 tests to determine which ones work best.
Disruptions in international transportation caused by the pandemic have also made it more difficult to ship testing supplies in a timely manner. Diagnostic laboratories have been overwhelmed with the volume of COVID-19 testing, sometimes causing testing for other diseases to fall by the wayside and testing for the new coronavirus to fall behind schedule.
Why testing will be important even with a vaccine
As countries begin to lift COVID-19-related activity restrictions, diagnostic testing remains extremely important. Identification of people infected with SARS-CoV-2 will help guide treatment of those individuals and indicate who needs to be isolated, and have their contacts traced and quarantined, to prevent the further spread of the disease. Widespread increases in COVID-19 cases confirmed through diagnostic testing will indicate if COVID-19-related activity restrictions need to be reimposed. If sensitive surveillance for COVID-19 is in place, an absence of such an increase after restrictions are lifted will provide increasing confidence that countries can safely continue to operate without those restrictions.
Even after a safe and effective COVID-19 vaccine is developed and made widely available, diagnostic testing will be essential to make sure that such a vaccine is used as effectively and efficiently as possible. Surveillance data could help identify which parts of the population most need the vaccine - reliable laboratory confirmation diagnostic testing could indicate which populations are still at risk of the new coronavirus, and which have already been fully infected and recovered. When combined with preventive tools such as vaccines and physical distancing, COVID-19 diagnostic tests are an essential part of a multifaceted approach to defeating the pandemic.