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  • Let's assume you've had COVID-19.

  • You might believe you're immune,

  • in the clear to go about your normal life

  • without safety precautions.

  • But with scattered reports of recovered cases

  • testing positive again,

  • a lot of people are wondering if it's possible

  • to develop immunity at all.

  • So, can you catch COVID-19 twice?

  • The thing is, scientists just aren't sure,

  • because figuring that out

  • is a lot more complicated than you might think.

  • When a pathogen like a virus

  • manages to get into a human,

  • their body will signal an alarm.

  • This will start what's referred to

  • as the innate immune response

  • and consists of physical, chemical,

  • and cellular defenses against pathogens.

  • Often enough, this works, and the invader is killed,

  • but sometimes you need a second attack.

  • The adaptive immune response marshals

  • the special forces:

  • B cells and T cells.

  • B cells produce antibodies

  • that smother the specific pathogen

  • so it can't affect your body's cells.

  • Shane Crotty: So, basically, antibodies

  • kill virus outside of cells;

  • killer T cells kill virus inside of cells.

  • Narrator: So, once your body fights the pathogen,

  • what stops you from having to fight off

  • the same attack over and over?

  • Well, your body has a secret weapon

  • to protect you against any future attacks

  • from the same pathogen: memory cells.

  • Memory cells are

  • basically the specialized

  • T cells and B cells that stick around as guards.

  • That way, if you encounter the virus again,

  • the army will be readily waiting

  • to kill that invader instantly.

  • This is immunity.

  • A vaccine works on this basis by adding dead,

  • weakened, or fragmented parts

  • of a pathogen to your body.

  • Not enough to cause illness,

  • but enough to cause

  • your body to produce memory cells.

  • Crotty: Normally, when you have an infection,

  • it's a race between your immune system

  • and the infection.

  • But if you're vaccinated,

  • you've already done the race part.

  • Your immune system has already had time

  • to scale up and develop immunity.

  • Narrator: Typically, you would know

  • if you have immunity,

  • either from a vaccine

  • or knowing you previously had the illness.

  • For example, it's relatively straightforward to know

  • whether you've had chickenpox or not

  • because the symptoms are highly unique

  • and very easy to spot.

  • But let's presume you know you've had COVID-19.

  • You now have immunity and are safe, right?

  • To test this theory, one early study

  • infected monkeys with the COVID-19 virus.

  • They then waited till they tested negative

  • after the infection passed and tried to reinfect them.

  • When the monkeys didn't become reinfected,

  • researchers concluded that after one viral attack,

  • you would be protected from another.

  • But this isn't a golden ticket to thinking you're immune,

  • because the length and strength

  • of that potential immunity are unknown.

  • And where a disease lies on the spectrum

  • is influenced by two things,

  • memory cell death rate and virus mutation rate.

  • Memory cell death rate tells you at what rate

  • those memory cells may be lost over time.

  • The virus mutation rate can tell you if the virus

  • will mutate too quickly for your memory cells.

  • The more a virus mutates, the more unrecognizable

  • it becomes to your memory cells.

  • Determining where COVID-19

  • falls on this timescale

  • is a vital step in managing its spread.

  • One indication of how long its immunity might last

  • is to look at other coronaviruses.

  • Those who have contracted SARS-1

  • have been found to have immunity

  • for about two to three years,

  • and the same time frame has been seen

  • in other coronaviruses that can cause the common cold.

  • Yet early signs have shown

  • that this virus tends to mutate slowly.

  • But there's another indicator

  • into immunity strength and length:

  • the serology test.

  • Crotty: The serology testing is a blood test

  • for the presence of antibodies against

  • that specific virus or that specific disease.

  • Narrator: Importantly, these are tests that can be done

  • after you've recovered from symptoms.

  • Crotty: And you don't have to know exactly

  • when that person was infected.

  • And so that's a very powerful way to count

  • how many people have actually been infected,

  • whether they recognize the symptoms or not.

  • Narrator: These tests can measure

  • how many antibodies are in the sample

  • by looking at how they block or respond to the virus.

  • These measurements can help to understand

  • immunity levels and how long immunity could last.

  • For example, studies for many other viral infections

  • have found that

  • the more severe the case,

  • the longer the immunity.

  • Basically, the bigger the infection,

  • the bigger the immune response

  • and more antibodies in a sample,

  • which in turn gives longer immunity.

  • But this may not hold true for COVID-19.

  • Crotty: It certainly may not be as simple as

  • if you're positive for the antibody,

  • you're protected against the disease, you're immune.

  • That's true for many infections;

  • it's not proven for COVID-19 disease.

  • Narrator: And the effectiveness of certain

  • serology tests for COVID-19 has been mixed.

  • Some tests are being misused,

  • and others were brought into the United States

  • before the FDA could approve them.

  • The result has been poor detection rates,

  • some as low as 20%.

  • Crotty: You know, these are the same types of tests

  • as a pregnancy test.

  • And so there's no way people would take pregnancy tests

  • if they were only accurate 60% of the time.

  • Narrator: And there have been instances

  • of false positives, which can be extremely dangerous,

  • because they arm people with

  • a false sense of potential immunity.

  • But these problems aren't universal,

  • and the FDA has begun approving a select few

  • that show much higher accuracy.

  • So, with an accurate serology test,

  • would you be immune?

  • Well, one early Chinese study

  • found 30% of those who tested positive for the virus

  • had little to no detectable antibodies,

  • which would suggest that immunity isn't guaranteed,

  • though this has been challenged by another study

  • that found all patients tested

  • had significant antibody levels.

  • But other issues such as age or health

  • could play into these responses.

  • Crotty: Is it gonna be 0.1% of people

  • who can get reinfected three months later?

  • Or is it going to be a higher number in the elderly?

  • Narrator: Once more accurate and universal

  • testing is underway,

  • more studies can begin to more precisely examine

  • how long this immunity may last and who has it.

  • So our best bet right now is to keep our distance

  • and assume we're not immune at all.

Let's assume you've had COVID-19.

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Can You Get The Coronavirus Twice?

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    林宜悉 posted on 2020/12/23
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