Five Minute Science Lesson to explain Covid-19 testing (but also other infectious diseases)
“So confusing! There’s one test that costs 4500 rupees and there’s another test that might cost a few hundred rupees. How?”
“One is an antigen test which is a diagnostic test. The second is the antibody test, that one is easier and cheaper to do, but gives you information more from a surveillance perspective. Does that make sense?”
“Ummm, I want to say yeah, because otherwise you’ll go into a long gyan session, and I have a long work day ahead...”
“Okay, just one quick question. Do you know what antigens and antibodies are?”
“I think I know what an antibody is. Something that the body produces to protect you from an infection? No clue what an antigen is.”
After this conversation with one of the most well-informed people I’ve met, I decided to make a quick science primer for all those friends and family who might have similar questions. It will take 5 minutes to read, there are only two new words you’ll need to understand, and you’ve already heard them a million times before. And then the science articles flying around will make sense.
Five minutes. Two definitions. I promise. And then not only Covid-19, a lot of other infectious disease debates will make sense. Polio the great success story. TB the secret pandemic. Dengue the annual surprise.
Definition 1: Antigen. Antigens are the “invading agents”. Think bacteria, viruses, protozoa, fungi , donor blood cells— basically all things foreign to your body which can potentially harm your body. Old ones like the TB bacteria, new ones like the Covid virus. Any part of an invading agent’s body can be the antigen.
Only one more definition: Antibody. Antibodies are human cells responsible for mounting defence against antigens. When any invader antigen enters your body, your body’s immune system responds and starts making specific antibodies to destroy or at least suppress this specific antigen. That is important so I’ll say it again — specific antibodies get created in response to specific antigens.
Antibodies can float around in your blood for some time (days or weeks or years) after they have destroyed the antigen. The body’s logic is: just in case this same antigen comes back, I won’t have to make this particular set of antibodies again- these floating ones are already experienced in destroying this antigen and they’ll do the job again. Vaccines use this logic — we give the body a small, controlled dose of the antigen, triggering the production of antibodies which give long term protection. The polio drops that we give to all Indian babies? That is a small dose of a weak polio virus, in response to which the body produces polio antibodies and sets up a solid wall of protection against polio.
That’s it. Here’s a picture: Purple,Y shaped antibodies surrounding and killing green antigen circles.
With this, two ongoing debate issues should become clearer:
1 — Types of testing: We can test for antigen and antibody both, and it serves different goals. First, there’s the antigen test — this one will find out if your body is carrying the antigen right now. This can have many names like PCR testing or DNA testing. This test is a definitive diagnostic test which basically means that if you test positive, you have the virus (in case of a disease like Covid) or the bacteria (in case of a disease like TB). Antigen tests are generally more expensive because antigens are tough to detect, especially in the early stages of infection — which is ideally when you want to diagnose the disease, because early diagnosis means better treatment outcomes. To increase your chances of finding the infecting antigen, you need to take samples from the site of infection, i.e. where the antigen where is most likely to be hiding. That is why we take throat swabs for covid, and sputum samples for TB. Not easy for anyone — neither the patient, nor the healthcare worker.
Then there is the antibody test, which is usually cheaper and quicker than antigen testing because antibodies are easier to detect. They’re happily floating around in the blood so all you need to do is test some blood. Not that getting blood drawn is fun, but it’s certainly easier than being asked to cough out sputum from the deepest reaches of your lungs, or getting your throat swabbed, trust me. Antibody tests are also called serology tests (from the word ‘serum’, which is part of blood). What does a positive antibody test tell me? That the antigen had entered my body at some point — it could be inside me right now, or it could have been a few years ago. Antibodies float around after the antigen is gone, remember? A positive antibody test means that at some point, my body managed to defend me against the invading antigen, at least for now. This test is very interesting from a public health perspective: if we randomly test 100 healthy people and 70 of them test positive for Covid antibodies, it means that 70 people managed to fight the infection. But let’s hold off on the celebrations for a bit. Antibody protection can last as little as a few weeks. We don’t know yet how long the Covid antibody protection lasts.
2 — Infection vs disease: Getting Covid infection is not the same as Covid the disease. When the Covid virus enters your body, you’re “infected” with the virus. But you might not get “sick” because your body made the antibodies and destroyed the virus before it could cause disease by attacking your body cells (there’s a lot of debate about which body cells get attacked but that can be a another lesson). So you might be infected with Covid (or TB) not know it. At any point in time, just over one third of Indians are infected with TB. That’s right, one out of three people. But “only” about 1% percent (which is over 3 million people) of us progress to the actual disease. Here’s another picture:
Why am I comparing Covid and TB? Well, because I work in TB, but more importantly, because they’re both pandemics. As of early May 2020, about 270,000 people have died of Covid. In the same four month period, nearly 400,000 people died of TB. Oops, my TB activism crept into my science lesson. This happens a lot.
