New immunity insights a boost against Covid-19
In a study, the researchers tested 15 people who had Covid-19, once between the first 20 days since they had the disease and for a second time around 86 days later, when the researchers found stable levels of B and T memory cells.Updated: Aug 19, 2020, 16:00 IST
Does getting Covid-19 once give the patient immunity to the disease? If so, how long does it last? Is the immunity robust enough to fend off the pathogen even in a highly contagious space? And, does the level of protection vary depending on how strong or mild the infection was?
At least four studies over the past few weeks have offered deeper insights into these crucial questions about how the body’s immune system responds to the Sars-Cov2 once it has been infected by and cleansed of it.
The studies, to answer the questions above in order, indicate that -- yes, a person once infected may be immune to reinfection; this protection may last for at least 3-6 months; the immune system is likely remember the virus to offer protection in a highly infectious situation; and the level of protection may be strong even if the infection was mild.
These answers are crucial because they will determine if vaccines will work to bring about a collective immunity that will stay long enough to return the world to the pre-pandemic period.
Research suggests the pathogen behaves in a manner similar to other viruses, allowing the human body to identify it, fight it, and remember it, irrespective of the nature and intensity of the illness.
The first indication of this was in a study published on July 24 by researchers from Wuhan, who followed the first of 349 people to be infected and develop symptoms in the central Chinese city where the virus was discovered. “Our data indicates sustained humoral immunity in recovered patients who suffer from symptomatic Covid-19, suggesting prolonged immunity... titers (antibody levels) stabilised at relatively high levels over the six months observation period,” said the researchers from Huazhong University of Science and Technology, Wuhan.
The second was submitted to the journal Cell on August 11 by researchers from Sweden, Denmark and the UK, who – in an analysis of samples from people in European hot spot areas – showed that people “exhibited robust memory T-cell responses months after infection, even in the absence of detectable circulating antibodies specific for Sars-CoV-2”. This, they added, indicated “a previously unanticipated degree of population-level immunity against Covid-19”.
The third came on August 14, in a study by University of Washington and Seattle’s Fred Hutch Cancer Research Center. It found that having antibodies (they had likely recovered from a previous disease) protected three crewmembers while the virus infected 104 of 122 people on a fishing vessel over an 18-day voyage. “This suggests that neutralising antibodies are a correlate of protection for Sars-CoV-2,” Alexander Greninger, the assistant director of UW Medicine Clinical Virology Laboratory told HT on Sunday.
The latest was reported on August 15, offering a second confirmation: people with mild symptoms had memory cells that could recognise the virus later. “We found that recovered individuals developed Sars-CoV-2-specific IgG antibody and neutralising plasma, as well as virus-specific memory B- and T-cells that not only persisted, but in some cases increased numerically over three months...” said researchers from University of Washington’s department of immunology.
In this study, the researchers tested 15 people who had Covid-19, once between the first 20 days since they had the disease and for a second time around 86 days later, when the researchers found stable levels of B and T memory cells.
T- and B-cells and antibodies are among the two main features of the complex immune system. While antibodies bind to viruses and neutralise them, T-cells and B-cells are responsible for producing antibodies, creating “helper” cells, and turning into killer proteins that gobble up any cell that is infected. Once an infection is beaten, 90% of T-cells and B-cells die off, while the remaining linger on as memory cells that can quickly multiply into antibodies, helper cells and killer cells.
While they are yet to be peer-reviewed, the first, second and fourth papers establish that the immune defences behave like they should with the Sars-Cov-2, while the second offers the closest sign yet that these may be enough to prevent a subsequent infection. UW professor Greninger, however, added that more studies with bigger groups may be needed to conclusively determine that antibodies can prevent reinfection.
Experts said that in addition to these studies, circumstantial evidence from China, where the outbreak first began around 9 months ago, too points to long-lasting immunity.
“Immunity must be quite long because China is not reporting reinfection cases. They admitted the problem in December but there are signs that it may have started in August. So the people who got infected in this time do not appear to be getting it for a second time,” said T Jacob John, the former head of virology department at Christian Medical College, Vellore.
John added that he believes that “both kinds of antibodies – binding as well as neutralising” need to be studied, and that T memory cell levels are expected to stay on for long.
The ability for the immune system to succeed against the virus and remember it for future encounters is crucial since there have been past viruses that interfere with the very defences of the human body. The virus that causes measles, for instances, has the ability to wipe out memory cells and make people vulnerable to infections they have had in the past. The dengue virus is known to cause a complication in which binding antibodies are not enough to neutralise it, leading to a situation where it can lead to severe infection. The human immunodeficiency virus (HIV) goes straight for the immune system.
Researchers involved in the above studies said this success of the immune system is crucial for vaccines to work. “The development of vaccines against Sars-CoV2 would be greatly facilitated by the identification of immunological correlates of protection in humans,” the researchers involved in the fishing vessel outbreak study noted, before elaborating their findings that showed that antibodies correlated with protection from infection.