Immunity 101 for COVID-19July 18, 2020 0 By FM
As antibody testing becomes more common, new studies have been released including one showing that some patients from Wuhan infected by COVID-19 did not develop antibodies as well as another tracking rapid loss of antibodies in both asymptomatic and symptomatic cases. This seems counterintuitive since the ability to develop immunity to a virus forms the basis of the science behind vaccines as well. However, equating the presence of antibodies with immunity ignores nuances that are vital to how our body protects us from viruses like COVID-19.
To start, there are actually two types of immunity — innate and adaptive. Innate immunity includes barriers like skin, hair and mucus as well as phagocytes, which indiscriminately gobble up foreign particles, and natural killer cells that are the first line of defense against infection.
When the body needs to mount a more targeted response against an invader like the virus behind COVID-19, then adaptive immunity kicks in. There are two types of white blood cells, called B-cells and T-cells, which play a role. While B-cells are responsible for creating antibodies targeting the virus itself, T-cells take a specialised approach to destroying already infected cells. These T-cells look for epitopes, or protein signatures, on a cell’s surface which identify it as infected. A study from Germany has found that T-cells which offered protection from older coronavirus can strengthen the immune response to COVID-19. This would explain why some people were able to recover from COVID-19 but then do not produce antibodies. Essentially, if you could get sufficient protection from your T-cells, then your B-cells never have to create antibodies.
But if your T-cell defences are not enough, then the IgG antibodies produced by B-cells circulate in plasma and neutralise pathogens directly or further activate agents of the innate immune system like phagocytes towards a particular target. While the antibody production may decrease sooner in some individuals, most of those who generated antibodies in response to an infection will retain them for a period. A mathematical model referred to in the same study showing loss of antibodies in patients from Wuhan predicts a duration of immunity as long as 45 weeks or roughly 11 months. That period of immunity is on par with what has been observed in cases of related coronaviruses, including SARS and MERS, but does not take into account resistance from T-cells or even the ability of memory B-cells to ramp up antibody production when they re-encounter a virus.
While lack of antibodies in some individuals and waning levels in others might make it difficult to offer antibody passports for COVID-19, it is necessary to fully understand all types of immunity in order to capture the risk within a population. It may not be possible to offer widespread T-cell tests, but sampling groups from different cities could show where T-cell resistance already exists and thereby enable relaxing of lockdown restrictions in those areas. While we wait for the vaccine, we need other solutions to mitigate the effects of COVID-19 lockdowns so that people can start to return to a level of normalcy. Even more critically, the immune response will impact the effectiveness of vaccines under development. Technologies that rely more heavily on generating antibodies, like using mRNA, may not have long term effectiveness compared to the older vaccination approach of using a weakened version of the virus.
The author has a Master’s in Biotechnology from the University of Pennsylvania.