Reviewed by ADEZE OJUKWU
COVID-19 has continued to raise anxieties and questions.
The World Health Organization (WHO) has provided some answers to some of these burning questions.
‘The answers to the questions below are based on the current understanding of the SARS-CoV-2 virus and COVID-19, the disease it causes.’
COVID-19: What is herd immunity?
‘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.
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.
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.
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.
Does the presence of antibodies mean that a person is immune?
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.
What is serology?
‘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.
What is the difference between molecular testing and serologic testing?
‘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.