COVID-19: Serology, antibodies and immunity

‘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.   

There are now more than 200 peer-reviewed publications,  pre-prints,  manuscripts and government reports of SARS-CoV-2 seroprevalence studies. These studies vary in study design, populations under study, serologic tests used, timing of sample collection, and quality. Overall, the population-based seroprevalence reported across available studies remains low, at below 10%.

Some studies conducted in areas of known high virus transmission and studies of health care workers in areas of known high transmission have reported seroprevalence estimates over 20%.

Available study results indicate that, globally, most people remain susceptible to SARS-CoV-2 infection.

‘Herd immunity’, also known as ‘population immunity’, is a concept used for vaccination, in which a population can be protected from a certain virus if a threshold of vaccination is reached.

Herd immunity is achieved by protecting people from a virus, not by exposing them to it. Read the Director-General’s 12 October media briefing speech for more detail.

Vaccines train our immune systems to develop antibodies, just as might happen when we are exposed to a disease but – crucially – vaccines work without making us sick. Vaccinated people are protected from getting the disease in question. Visit our webpage on COVID-19 and vaccines for more detail.

As more people in a community get vaccinated, fewer people remain vulnerable, and there is less possibility for passing the pathogen on from person to person. Lowering the possibility for a pathogen to circulate in the community protects those who cannot be vaccinated due to other serious health conditions from the disease targeted by the vaccine. This is called ‘herd immunity’.

‘Herd immunity’ exists when a high percentage of the population is vaccinated, making it difficult for infectious diseases to spread, because there are not many people who can be infected. Read our Q&A on vaccines and immunization for more information.

The percentage of people who need to have antibodies in order to achieve herd immunity against a particular disease varies with each disease. For example, herd immunity against measles requires about 95% of a population to be vaccinated. The remaining 5% will be protected by the fact that measles will not spread among those who are vaccinated. For polio, the threshold is about 80%.

Achieving herd immunity with safe and effective vaccines makes diseases rarer and saves lives.

Find out more about the science behind herd immunity by reading our dedicated Q&A or watching or reading this interview with WHO’s Chief Scientist, Dr Soumya Swaminathan.