New Delhi, November 21, 2020 :
‘Serology’ is the study of antibodies in blood serum.
‘Antibodies’ are part of the body’s immune response to infection. Antibodies that work against SARS-CoV-2 – the virus that causes COVID-19 – are usually detectable in the first few weeks after infection. The presence of antibodies indicates that a person was infected with SARS-CoV-2, irrespective of whether the individual had severe or mild disease, or no symptoms.
‘Seroprevalence studies’ are conducted to measure the extent of infection, as measured by antibody levels, in a population under study. With any new virus, including SARS-CoV-2, initial seroprevalence in the population is assumed to be low or non-existent due to the fact that the virus has not circulated before.
‘Molecular testing’, including polymerase-chain reaction (PCR) testing, detects genetic material of the virus and so can detect if a person is currently infected with SARS-CoV-2.
‘Serologic testing’ detects antibodies against a virus, measuring the amount of antibodies produced following infection, thereby detecting if a person has previously been infected by SARS-CoV-2. Serologic tests should not be used to diagnose acute SARS-CoV-2 infection, as antibodies develop a few weeks after infection.
When a new disease, like COVID-19 emerges, initial surveillance and testing strategies focus initially on patients with severe disease and the use of molecular testing to measure acute infections, as these are the individuals who seek and require health care. This can often miss the fraction of mild or asymptomatic infections that do not require medical attention, and as such, the full extent of infection is not known early in an outbreak.
Serologic testing helps retrospectively determine the size of an outbreak or extent of infection in a population under study. Seroprevalence studies give a more complete picture of how much of a population has been infected with SARS-CoV-2 and will capture unrecognized cases not identified through routine or active surveillance.
There are many studies underway to better understand the antibody response following infection to SARS-CoV-2. Several studies to date show that most people who have been infected with SARS-CoV-2 develop antibodies specific to this virus. However, the levels of these antibodies can vary between those who have severe disease (higher levels of antibodies) and those with milder disease or asymptomatic infection (lower levels of antibodies). Many studies are underway to better understand the levels of antibodies that are needed for protection, and how long these antibodies last.
To date, there are some reports of individuals who have been reinfected with SARS-CoV-2. There are likely to be more examples of reinfection reported and scientists are working to understand the role of the immune response in the first and second infection. WHO is working with scientists to understand each occurrence of reinfection and the antibody response during the first and subsequent infections.