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.