The first case of COVID-19 was reported back in December 2019. Fast forward more than 9 months, and there is a lot still unknown about the disease. We know that COVID-19 spreads through respiratory droplets but we don't know for sure how this disease emerged in the first place or if it can indeed spread through the air. Similarly, while immunocompromised people and those with chronic conditions are at high risk of severe disease, reports suggest that otherwise healthy people can also die of it. And then there is the curious case of immunity.
Studies show that people with the disease develop antibodies against SARS-CoV-2, the causative agent of COVID-19. However, a lot of people have shown only low levels of neutralising antibodies and cases of reinfection have also been noted in some parts of the world.
On the other hand, antibodies against SARS-CoV-2 have been found in otherwise unexposed people, pointing to some sort of cross-reactivity with other related viruses.
All this data raises a lot of questions - who is actually immune to the disease and for how long - to name a few.
Serological data and herd immunity
Experts suggest that there are two main ways a pandemic can end - enough people get the disease and we generate herd immunity or the disease is eliminated with mass vaccination. A third way is to learn to live with the disease and follow proper precautionary measures to contain the spread.
While vaccine candidates are still in their development phases, serological data from Spain and Sweden suggests that herd immunity may not be a viable option either. Even without a timely lockdown and more cases and fatalities than other Scandinavian countries, Sweden only had 15 percent herd immunity by May 2020 as compared to the projected 40%. Serological testing in Spain early in July showed only 5 percent herd immunity, most of which was in the worst-hit areas like Madrid.
It has previously been suggested that immunity to COVID-19 may not last for more than six months and even if a vaccine comes out it may have to be taken twice a year. However, the cross-reactivity with common cold causing coronavirus antibodies and the resultant reduction in infection severity raises some hopes.
The complexity of the immune system
Over the last few months, several studies have indicated the presence of robust T-cell immunity against the COVID-19 causing virus in people even after their antibodies were depleted.
The human immune system is a complex machinery consisting of a variety of cells and compounds that work in tandem to fight a pathogen. Antibodies protect you from a microbe present outside the cells (like in blood) while T-cells handle intracellular pathogens like viruses.
Cytokines like interleukins help maintain communication between the various immune system cells so they can work together. A study done in the USA suggested that children have an entirely different response to COVID-19 and hence they don't get severe disease. Specifically, they have a stronger initial response to the virus including cytokines like IL-17A and IFN-gamma while the adult body depends on antibodies, which are formed much later.
Dr Karl Friston, a professor of imaging neuroscience at University College London has pointed out that there is some sort of preexisting immunity in a lot of people against COVID-19. He used the term immunological dark matter to explain this unknown. Though a lot of experts have criticized the concept, some researchers have indicated that we should look beyond antibodies to check for immunity against COVID-19.
Besides, some of the cases of reinfection that have been seen in the world have been from a different strain of the virus, which our immune system may take as an entirely new pathogen - different strains of flu do cause recurrent infections. Though researchers suggest that reinfection is rare, given only a few cases of it in the millions of confirmed COVID-19 cases worldwide. However, the concept of reinfection needs to be studied further to see any other possible causes.
For more information, read our article on Immunity to COVID-19.
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