COVID-19 Immunity: Lancet study suggests herd immunity may not be possible, antibodies found only in 5% of test population in Spain
Herd immunity of importance as for the pandemic to be over, enough people in a community have to develop immunity against the coronavirus either after getting the disease or through vaccination
Herd immunity is a term that most of us have become acquainted with during the coronavirus pandemic. The concept is of importance as for the pandemic to be over, enough people in a community have to develop immunity against the disease either after getting the disease or through vaccination.
Now, a population-based seroepidemiological study done in Spain (ENE-COVID), indicates that only 5 percent of the population in the country has antibodies against COVID-19. While no difference was found in the antibody levels of both genders, more people were found to have immunity against the disease in the hotspot areas of Madrid (greater than 10 percent) than those living in the coastal areas where the disease was not as prevalent (less than 3 percent).
The Lancet study suggested that the prevalence of antibodies in the general population is very low and not sufficient to provide herd immunity.
Findings of the study
The study involved more than 61,000 people from across the country and is suggested to be the largest European study to check the seroprevalence of SARS-CoV-2 antibodies so far.
For the study, all the individuals were tested twice - once with a point of care rapid test and again with a lab-based immunoassay test - for the presence of IgG antibodies in their blood. IgG antibodies are formed later in an infection and are responsible for long-term immunity.
Both the tests looked for different antigens on the surface of SARS-CoV-2 virus. The seroprevalence (number of people who had antibodies against the virus in the population) was found to be in the range of 3.7 percent and 6.2 percent depending on if we look for specificity (positive result in both tests) or sensitivity (positive result in one of the tests).
Here are some other findings of the study:
- Antibody levels were low in people above the age of 85 and children below the age of 10. The latter was suggested to be due to less ACE-2 receptors in the nasal mucosa.
- Essential healthcare workers had almost the same amount of antibodies as the general population. However, compared to those who never came in contact with a confirmed COVID-19 case, those who lived in the same household as a COVID-19 positive person or who came in contact with such people say at a workplace had higher antibody levels. The study indicated that this could be due to the spread of the virus in closed spaces, since physical distancing within a house is virtually impossible in an urban setting.
- Healthcare professionals did have a higher seroprevalence but that is suggested to be due to their higher exposure to the virus. Healthcare workers comprised about 24 percent of all COVID-19 cases in Spain.
- About 90 percent of the people who had symptoms showed antibodies within two weeks of symptom onset.
- Of all the people who reported having COVID-19 like symptoms, only 16-20 percent of people had gotten a PCR test, though about 79 percent of them had tested positive. Only 15.3-19.3 percent of symptomatic people had COVID-19 antibodies, pointing to the fact that some of their symptoms might have been due to other conditions.
Reliability of rapid tests
Point of care tests are highly effective in giving quick results and can be helpful in testing at the patient’s bedside. However, one of the major questions regarding rapid testing is reliability.
The study indicated that even though immunoassays are said to have better performance, rapid tests are as effective and gave comparable results in epidemiological tests. Since the two tests look for different antigens, the combination might be a good way to check for a wider range of antibodies in people.
The rapid test looked for antibodies against the receptor-binding domain of the COVID-19 causing virus, while the immunoassay looked for SARS-CoV-2 nucleoprotein or N protein. The receptor-binding domain is what the virus uses to bind to the ACE-2 receptors on healthy cells while the Nucelocapsid protein is a conserved protein in coronaviruses which helps form the outer coat of the virus and plays a role in viral release from the infected cell.
For more information, read our article on What is herd immunity.
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