Risk factors for few SARS-CoV-2 antibodies after infection

Disease and treatment 16. jan 2022 2 min Professor Susanne Dam Poulsen, Clinical Assistant Omid Rezahosseini Written by Kristian Sjøgren

Obesity and mild illness are risk factors for a weak antibody response to SARS-CoV-2 infection, which likely increases the risk of reinfection.

Some people do not have detectable levels of SARS-CoV-2 antibodies after infection (seronegative) and probably therefore do not have a strong immune response against reinfection.

Researchers from Rigshospitalet screened healthcare workers in the Capital Region of Denmark to determine who is at higher risk of establishing a weak antibody response to SARS-CoV-2 after infection. They found that this especially applies to people with obesity or mild illness.

The researchers also determined the percentage who are at higher risk, and the result can be used to identify people that may require special attention in future COVID-19 surges.

“We want to identify the people who we need to keep an eye on. Many people infected with SARS-CoV-2 think that they are protected against reinfection. However, if they have not established a high level of antibodies, they are probably not as well protected as they think. We identified the risk factors for being seronegative after infection and the percentage of the participants to whom this applies,” explains an author behind the study, Susanne Dam Poulsen, Consultant and Professor, Department of Infectious Diseases, Rigshospitalet, Copenhagen.

The research has been published in Microbiology Spectrum.

Antibodies protect against reinfection

Antibodies against viruses are not always that complicated. People infected with SARS-CoV-2 create antibodies, and these remain after the virus is gone, protecting against reinfection.

However, people differ in their antibody responses. Some people make many antibodies, others only a few, and a third group is seronegative and probably less well protected from reinfection.

“This applies both to previous infection and vaccination. Developing antibodies helps to protect against infection. This happened in the first wave of infection in Denmark in early 2020. The people infected then were 80% protected from reinfection in the second wave,” says another author behind the study, Clinical Assistant Omid Rezahosseini from the Department of Infectious Diseases at Rigshospitalet.

A few are seronegative

In the new study, the researchers wanted to determine whether people who were seronegative after SARS-CoV-2 infection had any special risk factors and therefore analysed data on 36,583 healthcare workers.

The participants had blood sampled three times to determine whether they were seropositive or seronegative for SARS-CoV-2 antibodies.

A total of 866 of the participants had a positive result on the polymerase chain reaction (PCR) test before or during the study period. The average age for these participants was 42 years, and 77% were women.

Further studies of the serostatus of those testing positive on the PCR test revealed that 2.4% were seronegative 4 months after testing positive.

“The vast majority create antibodies, which is good because it indicates protection against reinfection,” explains Susanne Dam Poulsen.

People with obesity and mild illness produced few antibodies

In the next step, the researchers examined any specific characteristics of those who did not develop a sustained SARS-CoV-2 antibody response.

Omid Rezahosseini says that classical risk factors for not establishing a good immune response include being old and smoking.

The researchers therefore adjusted for these known risk factors and found two additional risk factors associated with being seronegative: obesity (body-mass index exceeding 30) and mild illness. More of the seronegative participants (2.4% of the total) had these risk factors than the seropositive participants.

Obesity was associated with a 3.1-fold increase in the hazard ratio for being seronegative, and mild illness was associated with a 6.6-fold increase.

“This indicates who should get special attention in future COVID-19 surges, even though they have already had COVID-19. Further, these are healthcare workers, who are the most susceptible to becoming reinfected because of their exposure. These are the important messages,” says Omid Rezahosseini.

Susanne Dam Poulsen emphasises that this information was especially important before vaccines were available, but the results are similar for vaccination, with individuals with obesity also seeming to have a higher risk of a weak antibody response after vaccination.

“These are the same risk factors that should be considered,” she concludes.

​The Department of Infectious Diseases provides specialist treatment and research within infectious, tropical and immunologic diseases.

​The Department of Infectious Diseases provides specialist treatment and research within infectious, tropical and immunologic diseases. Overall aim is...

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