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Written by Royal Melbourne Hospital's Suellen Nicholson, Head of the Infectious Diseases Serology Laboratory at the Doherty Institute.

Laboratory testing for SARS-CoV-2, the virus that causes COVID-19 disease, is vital to identify who is infected so they can be treated, isolated and help contact tracing to limit community transmission of the virus. We hear daily about the number of positive tests and total number of tests performed.

These are molecular PCR (polymerase chain reaction) tests, detecting SARS-CoV-2 virus RNA (part of the virus) from nasal or throat swabs. However there is a second type of test that detects antibodies to the virus.

After infection with a virus the majority of people will develop an immune response which includes producing antibodies to the infecting virus. These antibodies are detectable in a patient’s blood using serological tests, and identify people who have been infected with the virus.

There are pros and cons of PCR and serological testing

PCR tests are positive very early on in the infection and become negative with clearance of the virus by the patient’s immune system. In contrast antibodies are produced over days to weeks after infection with the virus, and serology remains positive once you have been infected. Long-term studies are required for COVID-19 for us to understand how long these antibodies last.

Serology and PCR have complementary roles in diagnostic testing. Very early in an infection, the PCR will be positive but serology is likely to be negative. A sample collected later in the infection when the virus is no longer present, is more likely to be PCR negative and serology positive. Test choice is determined by the reason for testing and timing after infection.

Why are serological tests important?

Serology testing is critical for understanding the extent of virus transmission in the community, so we know how many people have been infected, which in turn helps inform governments and health officials to guide their response to the pandemic and protect the community’s health. Antibody tests are particularly important for detecting previous infection in people who had mild or no symptoms, and who would not have undergone PCR testing.

It enables us to determine the proportion of people infected who will suffer fatal disease and the proportion of the population who may be protected against infection in the future. Antibodies are also produced after vaccination. Serology can be used to detect these antibodies as an indication of protection.

What’s currently being done at the Doherty Institute?

Different types of antibodies are produced by the human body. Detection of some types indicates recent infection with a particular virus and others past infection. Whether antibodies provide immunity or not varies from one virus to the next.

It is likely that some of the antibodies, known as neutralising antibodies, resulting from infection with the SARS-CoV-2 virus will provide protection from re-infection with the virus, however this still needs to be proven. If we determine which antibodies do provide immunity, we will also need to know what level of antibody is required to protect someone from reinfection, and how long this antibody will remain protective.

The Doherty Institute is leading some major public health, discovery, translational and clinical research responses to COVID-19 in collaboration with other institutes, hospitals and the private sector. Laboratory groups within the institute are specifically working on:

• Understanding the antibody response over time.
• How antibodies and other aspects of the immune system interact in COVID-19 infection.
• Investigating which serology tests give the most accurate results; including laboratory-based and rapid, point-of-care Tests.
• National Serosurvey studies to determine what proportion of the population have been infected with SARS-CoV-2 virus.
• Vaccine studies to determine the effectiveness of a vaccine by looking at the type and amount of antibody produced post vaccination.

How can we implement serology testing?

Serology testing for COVID-19 infection can be performed in accredited laboratories with trained personnel. These tests help determine if someone has been infected with COVID-19 or developed an antibody response post vaccination (when a vaccine is available).

However, it is important to remember we cannot currently assume, given our still limited understanding, that the presence of antibody is protective against reinfection with SARS-CoV-2 virus.

It’s wonderful to see how quickly we’re increasing our knowledge and understanding of COVID-19. The accelerated rate, a reflection of the overwhelming response at the local, national and global level to address the pandemic, as well as increasing our understanding and preparedness for future possible coronavirus disease outbreaks.