Covid-19 antibody response in most people strong, does not decline rapidly, study says

A vast majority of individuals infected with mild-to-moderate Covid-19 mount a "robust" defence via antibodies that is relatively stable for "at least five months", according to a new study which says this immune response significantly reduces the risk of re-infection with the novel coronavirus.

The research, published in the journal Science, found that this antibody response correlates with the body's ability to neutralise the SARS-CoV-2 virus that causes Covid-19.

"While some reports have come out saying antibodies to this virus go away quickly, we have found just the opposite -- that more than 90 per cent of people who were mildly or moderately ill produce an antibody response strong enough to neutralise the virus, and the response is maintained for many months," said Florian Krammer, a senior author of the study from The Mount Sinai Hospital in the US.

"Uncovering the robustness of the antibody response to SARS-CoV-2, including its longevity and neutralising effects, is critically important to enabling us to effectively monitor seroprevalence in communities and to determining the duration and levels of antibody that protect us from reinfection," Krammer said.

To screen individuals, the scientists used an antibody test called an enzyme-linked immunosorbent assay (ELISA) to detect the virus' telltale spike protein which enables it to attach and gain entry into our cells.

In the study, the researchers set up antibody test results using distinct dilutions of patient serum set at 1:80, 1:160, 1:320, 1:960, or 1:2880 and higher levels (titres).

They explained that these scores are generated by the number of times the scientists can dilute a patient's serum and still be able to detect the presence of antibodies.

Dilutions of 1:80 and 1:160 were categorised as low titres, containing low antibody levels, 1:320 as moderate, and 1:960 or 1:2880 categories as those with greater antibody levels.

Of the 30,082 positive samples, the study noted that 690 (2.29 per cent) had a titre of 1:80; 1453 (4.83 per cent) of 1:160; 6765 (22.49 per cent) of 1:320; 9564 (31.79 per cent) of 1:960; and 11610 (38.60 per cent) of 1:2880.

Based on the findings, the scientists said a vast majority of positive individuals had moderate-to-high levels of antibodies that can neutralise the coronavirus spike protein.

The research team then assessed 120 samples of known ELISA titres ranging from "negative" to those with 1:2880 and higher levels.

It found that approximately 50 per cent of sera in the 1:80-1:160 titre range had neutralising activity, 90 per cent in the 1:320 range could kill the virus, and all sera in the 1:960 to 1:2880 categories could block the SARS-CoV-2 virus.

The scientists then recalled 121 recovered Covid-19 patients at a variety of titre levels for repeat antibody testing at approximately three and five months post-symptom onset.

They saw a slight drop in the antibody levels in these individuals from the first to second testing time point, and another drop for the last testing time point.

According to the researchers, this indicated that a moderate level of antibody is retained by most people five months after symptom-onset.

In the higher titre range, they said there was a slow decline of antibodies over time.

The scientists noted that there was an initial increase in titre for individuals who had originally tested as having low to moderate titre levels.

"The serum antibody titre we measured in individuals initially were likely produced by plasmablasts -- cells that act as first responders to an invading virus and come together to produce initial bouts of antibodies whose strength soon wanes," said Ania Wajnberg, another co-author of the study.

"The sustained antibody levels that we subsequently observed are likely produced by long-lived plasma cells in the bone marrow. This is similar to what we see in other viruses and likely means they are here to stay," Wajnberg said.

According to the scientists, the vast majority of individuals with antibody titres of 320 or higher showed neutralising activity in their serum that are stable over a period of at least three months with only modest declines at the five-month time point.

Although this cannot provide conclusive evidence that these antibody responses protect from re-infection, the researchers believe the antibodies could decrease the odds of getting reinfected, and may attenuate disease in the case of breakthrough infection.