The number of Brits with CCP virus antibodies dropped by one quarter over a three-month period according to a large-scale study—but scientists say it doesn’t prove the case against stable natural immunity.
Researchers from Imperial College London used DIY finger-prick tests for antibodies on 365,000 people at home over three rounds.
In the first round, 6 percent had antibodies. Three months later, that had declined to 4.4 percent.
Dr. Helen Ward, one of the lead authors of the report said, “This very large study has shown that the proportion of people with detectable antibodies is falling over time.” She added, “we don’t yet know whether this will leave these people at risk of reinfection with the virus that causes COVID-19.”
Whilst antibodies are generally strongly associated with immunity, they don’t protect against every disease. Conversely, immunity against some diseases doesn’t depend on antibodies, but instead can rely on other mechanisms, including T-cells.
“It remains unclear what level of immunity antibodies provide, or for how long this immunity lasts,” Dr. Paul Elliott, director of the programme at Imperial, said in a statement.” If someone tests positive for antibodies, they still need to follow national guidelines including social distancing measures, getting a swab test if they have symptoms and wearing face coverings where required.”
The study did not follow individuals over time, but only measured overall numbers. However, according to Dr. Rowland Kao, professor of veterinary epidemiology and data science, “the size of the study and consistency of the results with other coronavirus infections are strong indicators that the measured decline is representative of the rate of decline for the population as a whole.”
But the fact that “substantial proportions” of people appeared to retain an immune response should help blunt the severity of any spread in the winter, he said in a statement.
Thirty percent of those testing positive had no symptoms. The research by Imperial used a simple DIY test kit that people used at home.
Some scientists say the study may indicate more about the sensitivity of that test than provide meaningful data on numbers of people with antibodies.
Dr. Alexander Edwards associate professor in biomedical technology at the Reading School of Pharmacy at the University of Reading said that these rapid tests are only able to detect high levels of antibodies compared to labs, and only give a binary result: positive or negative. Lab tests, on the other hand, can indicate a sliding scale.
“If you use a single cutoff value at a higher signal in these [lab] tests, this would potentially lead to the same conclusion as seen in [the Imperial study], i.e. fewer patients detected,” said Edwards in a statement. “When people are ill, antibody levels rise, and when you heal, antibody levels do drop naturally—this is not exactly the same as losing immunity.”
Dr. Eleanor Riley professor of immunology and infectious disease at the University of Edinburgh also notes the research may point to the limitations of those kits. “This study suggests that antibodies that can be detected by a simple DIY test may not persist beyond about 3 months after infection. If true, then these tests are unlikely to be useful for estimating cumulative population exposure to the virus or for individuals to use to assess their own previous exposure.”
Riley said it was too early to say whether immunity to COVID-19 does not last. “The study does not look at antibody concentrations, antibody function or other aspects of immunity such as T cell immunity and does not look at the trajectory of antibody levels in the same individuals over time.”
The fall-off in antibodies was higher with age, according to the Imperial study, and was found in all areas of the country. Only one category of people retained their antibodies over the three months: health care workers. This could be due to repeated or higher levels of exposure to the virus, the researchers say.