COVID-19 will still be with us for a long time, and the number of confirmed cases has been increasing in various parts of the world. If you think you are sure that you already had the virus, you probably think that you are now immune. But could you have COVID-19 a second time ?

Can you get COVID-19 again?

You probably think that once you have had COVID-19, which is caused by an infection with the SARS-CoV-2 coronavirus, it is unlikely that you will contract it again.

While this is the case for some viruses, such as chickenpox and measles, there are many other viruses that we can catch more than once. These include common colds, which can be caused by other types of coronaviruses that are capable of infecting people multiple times , and the flu.

The good news is that there have been very few cases around the world where COVID-19 has been contracted a second time.

We have already experienced several months of isolation thanks to this virus, and the number of cases in which people have contracted the virus for the second time is minimal. Only about four or five cases of reinfection have been heard across Europe.

Let’s take a look at why this happens.

COVID-19 antibodies could protect you from reinfection

The antibodies are molecules produced by special cells of the immune system (known as B cells) in response to an infection. These bind to viruses and prevent them from entering our cells.

Antibodies tend to stay in the blood for some time. So if a person is exposed to the same virus again, they can quickly generate an immune response. However, they don’t stay forever.

Research suggests that antibodies to the SARS-CoV-1 coronavirus, which caused the SARS pandemic in 2003, begin to disappear about three years after infection.

A small study carried out by UK researchers showed that antibodies to the SARS-CoV-2 virus can last for a few months. Which could explain why we haven’t seen many cases of reinfection so far. However, in some people, the antibody levels had started to return to normal after 60 days .

The big question here is, what happens when these antibodies start to disappear? Are you vulnerable to being infected with SARS-CoV-2 again?

It was believed that because some people do not seem to develop antibodies, or they disappear quickly after becoming infected, there would be many people who would become ill again. But that doesn’t seem to be the case. So immunity is about more than just antibodies.

Our immune system is made up of more than just antibodies

In addition to antibodies and the B cells that make them, we have another important weapon in our immune arsenal: T cells.

T cells have a variety of functions including protecting us against invading pathogens like bacteria and viruses , destroying infected cells, and controlling the production of antibodies by B cells.

After an infection, our immune system creates so-called memory T cells . As the name suggests, these cells ‘remember’ previous encounters with pathogens and respond quickly to repeated infections.

While antibody immunity often wanes relatively quickly over time, the resulting immunity from memory T cells can last much longer.

Memory T cells can also respond to viruses that resemble others that we have encountered before, even if we have never been infected with the new threat. This explains why a recent study has shown that between 20 and 50% of people in the US have their T-cell responses to SARS-CoV-2 even if they have never had it, which could offer true degree of immunity.

The latest research on immunity to COVID-19 suggests that T-cell responses may be much more important than we think. And even people exposed to recent cold viruses may receive some protection.

What is herd immunity about?

There has been a lot of talk about the concept of “herd immunity”. This is the idea that COVID-19 will no longer be a threat when enough of the population have developed immunity to the virus. Either by direct exposure or by vaccination.

Given that the possibility of a vaccine is still somewhat distant, is there any possibility that a significant proportion of the population could be protected by having already contracted COVID-19?

In a study by TwinsUK (a UK adult twin registry) around 400 of the participants living in the south east of England were tested, and 12% of them tested positive for COVID antibodies. 

However, about half of the people who have been infected may have had only a brief immune or antibody response. So the proportion of people who have some degree of immunity may be higher.

Although Twins UK participants may not be fully representative of the entire population. It is reasonable to assume that at least one in ten people in the south east of the UK may have already had COVID-19. This may explain why the second wave seems to be hitting the north of the UK now much more strongly than the south.

It could also be because immunity rates are higher in younger, fitter people who are now leaving home. Compared to older people and those with underlying health problems who tend to stay home.

This could be creating a “temporary immunity” by slowing down the rate of transmission and buying some time to flatten the increase in infections. But given how quickly rates are rising, it’s unclear how long it will last.

Still, researchers think that around 50-70% of the population needs to be immune to the coronavirus to benefit from long-term herd immunity. This is unlikely to be achieved anytime soon based on the number of people who have contracted COVID-19 so far.

More importantly, allowing COVID-19 to spread through the population uncontrollably would likely lead to the death of many tens of thousands of vulnerable older people and cause significant long-term health problems .

It’s worth noting that if some of us have pre-existing memory T cells, herd immunity thresholds will be lower and fewer of us will have to contract the disease to protect the entire population. But right now, it’s too early to tell.

By Dr. Eric Jackson

Dr. Eric Jackson provides primary Internal Medicine care for men and women and treats patients with bone and mineral diseases, diabetes, heart conditions, and other chronic illnesses. He is a Washington University Bone Health Program physician and is a certified Bone Densitometrist. Dr. Avery is consistently recognized in "The Best Doctors in America" list.

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