A recently published study by researchers at King’s College London shows SARS-CoV-2 antibodies fall to undetectable levels within 60 days in some patients. These findings potentially undermine the efficacy of vaccines aimed at triggering an antibody response, along with hopes of herd immunity.
The study, which involved periodically testing for SARS-CoV-2 specific antibodies, followed 96 patients in the UK who had tested positive for the virus for up to 94 days after symptom onset.
Symptom severity in the group ranged from asymptomatic to severe respiratory failure. Anyone that was not hospitalised was classified as having mild symptoms.
The study looked at three kinds of SARS-CoV-2 specific antibodies: IgM, IgA and IgG.
Overall, IgM and IgA antibodies declined fairly rapidly compared to IgG levels, which remained elevated for more than 90 days in the majority of patients, a common pattern for antibody responses.
However, when researchers looked at the results based on symptom severity they found that even the longer lasting IgG antibodies were undetectable 60 days after symptom onset in some patients who had experienced mild symptoms.
In general, those with mild symptoms had fewer antibodies at their peak, and those with fewer peak antibodies were more likely to end up with undetectable levels of antibodies after 60 days.
These results stand in contrast to the immune response to the earlier SARS coronavirus (SARS-CoV). In one study, patients with this virus had a peak antibody response after about 30 days and typically still had detectable levels of the longer lasting IgG antibodies 3 years later.
The antibody response to SARS-CoV-2 appears to be more like the response to seasonal coronaviruses associated with the common cold, which show a rapid decline in antibodies and frequent re-infection after 12 months, often without symptoms.
The King’s College study prompts a number of important questions. The first is whether SARS-CoV-2 infection leaves patients with any long-lasting antibody protection against the virus, something that might be critical for herd immunity. It remains unknown whether the levels of antibodies detected, even in those with detectable levels after two months, will continue to decline to a negligible level or flatline at some level that provides longer term resistance to the disease. Only time and follow up research will tell.
The second is whether antibody-triggering vaccines will provide durable immunity given the transience of the antibody response in some infected with the virus. Yesterday’s excitement following the early successful trial of such a vaccine produced by US biotech firm Moderna might turn into disappointment if its effects turns out to be fleeting.
This research also suggests that anyone who suspects they might have had a mild case of coronavirus in March or April and who hopes an antibody test might confirm that they’ve had it could be disappointed.
However, the authors of the study point out that there are other elements of the immune system beyond antibodies. Antibodies, which tag and attack pathogens, are only one part of it. T-cells, which are able to identify and kill cells infected by a pathogen, are another typically long lasting element of immunity. So it is possible those who have been infected, but have negligible antibodies, have some T-cell-based resistance to SARS-CoV-2.
Another study done in Sweden, which focused on T-cells, suggests that some people might have T-cell immunity to SARS-CoV-2, despite testing negative for antibodies. However, if T-cells turn out to provide resistance and immunity to the virus, it is still not clear whether they would prevent patients carrying and transmitting it, something critical for herd immunity.
This research and the questions it provokes are yet another reminder of how little we know about this new disease which has been with us for less than a year.
Kings College London study (in English)