Scientists are growing increasingly confident about the human immune response to SARS-CoV-2 after studies showed antibodies provide “real-world” protection against the virus and cellular immunity may be long-lasting, even in mild cases.
A study of a Covid-19 outbreak on a Seattle fishing boat involving more than 100 sailors has all but proven that antibodies provide protection against re-infection. Meanwhile, an encouraging body of evidence has found that T and B cells remain in the blood even once antibodies fade.
“This is exactly what you would hope for,” Marion Pepper, an immunologist at the University of Washington and an author on one of the new studies, told The New York Times. “All the pieces are there to have a totally protective immune response.”
“This is very promising,” echoed Smita Iyer, an immunologist at the University of California, Davis “This calls for some optimism about herd immunity, and potentially a vaccine.”
Antibodies have long been thought to protect against reinfection but the first study to use hard real-world evidence was published last week by researchers at the University of Washington.
It tracked the 122 strong crew of a fishing boat operating in the Pacific off the coast of Seattle, Washington. All were tested for both antibodies and the virus before they sailed and after. An outbreak occurred on the vessel and 104 people become infected - an attack rate of 85 per cent.
However, only those without pre-existing antibodies caught the virus. Of the three crew members who had already been exposed to the disease prior to the boat’s departure and had antibodies, none showed evidence of reinfection.
Professor Danny Altmann, of the department of immunology and inflammation at Hammersmith Hospital, Imperial College, said: "While this is a small study, it offers a remarkable, real-life, human experiment at a time when we've been short of hard-line, formal, proof that neutralising antibodies genuinely offer protection from re-infection. In short, it’s good news”.
While the findings are welcome news, key questions remain over the human immune responses to the virus - namely around how long such antibodies last. Most studies show they start to fade after just a few months.
However, there is mounting evidence that T cells and B cells, often described as “memory cells”, provide longer-lasting protection.
It was widely reported last month that researchers at the Karolinska University Hospital and University Hospital of Wales had found that people who recovered from asymptomatic or mild cases of Covid-19 may have long-term T-cell immunity against severe infection.
Similarly, another Seattle-based study published just last week found that patients who had recovered from mild forms of the virus had developed SARS-CoV-2-specific antibodies and neutralizing plasma, as well as virus-specific memory B and T cells.
These “memory cells” not only persisted but in some cases increased numerically over the three months following symptom onset.
According to the study, the T cells multiplied upon re-encounter with the virus, while the B cells expressed receptors “capable of neutralising the virus”.
As such, the authors claim that the findings demonstrate that even mild Covid-19 can elicit memory cells that provide protective immunity.