Coronavirus: what science already knows about post-covid immunity

A new study of people who have had Covid-19 and recovered from coronavirus raises the likelihood that immunity to the virus is short-lived.
Scientists at King's College London studied how the body naturally fights the virus through the production of antibodies, and how long these antibodies last in the weeks and months after recovery.
Almost all of the 96 people analyzed in the study had antibodies that could neutralize and stop coronavirus. But levels began to decline three months after the survey.
The possibility that the protection against covid-19 for those who have already caught the disease lasts a short time puts in check theses that indicate that regions that have been greatly affected by coronavirus and now register a drop in cases - such as Europe, USA and cities such as São Paulo and Manaus, in Brazil - are permanently protected.
Our immune system is the body's defense against infections, and is composed basically of two parts.
The first is always ready to act when any invader is detected in the body. It is known as the natural immune response and includes the release of chemicals that cause inflammation and white cells capable of destroying infected cells.
But this system is not specific to coronavirus. He won't learn, nor will he give you immunity against coronavirus.
Instead, adaptive immune response will be required, which includes cells that produce specific antibodies that can adhere to the virus to neutralize it and T cells that can attack only cells infected with the virus, the so-called cellular response.
This takes time - studies indicate that it takes about 10 days for the body to start producing antibodies that can attack the coronavirus and for sicker patients to develop a stronger immune response.
If adaptive immunity is strong enough, then it can leave a lasting memory of the infection, which will ensure protection in the future.
It is not known until now whether people who have only mild symptoms, or who have no symptoms, will develop a sufficient adaptive immune response.
Scientists are still working to understand the role of these T cells in the response to covid-19. But a recent study pointed out that people who tested negative for antibodies against coronavirus may still have some immunity.
For each person who tested positive for antibodies, the study found two that had specific T cells that identify and attack infected cells.
The memory of the immune system is like ours - clearly remembers some infections, but forgets others.
Measles, for example, is highly memorable - a single contact gives immunity for life (as do the weakened version of the viruses in the triple viral vaccine against measles, rubella and mumps).
There are many other infections, however, that are fully "forgettable". Children, for example, can catch the respiratory syncytial virus many times during the same winter.
The new coronavirus, called Sars-CoV-2, has not been known long enough to know how long immunity against it lasts, but there are six other coronaviruses that reach humans and can give a clue.
Four of them produce the symptoms of a common flu and immunity is short. Studies show that, in these cases, patients can be reinfected in less than a year.
In the case of the two other viruses - those that cause Severe Acute Respiratory Syndrome (Sars) and Middle East Respiratory Syndrome (Mers) -, antibodies were detected a few years later.
"The question is not whether or not you become immune, it's for how long," said Paul Hunter, a professor of medicine at the University of East Anglia. He adds, "It's almost certain that it won't last a lifetime," he says.
"Based on Sars' studies, it is possible that immunity will only last about a year or two, although it is not yet certain about it."
Even if you are not completely immune, however, it is possible that a second infection is not so severe.
Has anyone ever caught Covid twice?
There are preliminary reports of people who appear to have been infected more than once by the new coronavirus in a short period of time.
But the scientific consensus is that the issue was the tests, with patients being incorrectly informed that they were free of the virus.
No one was deliberately reinfected with the virus to test for immunity, but some rhesus monkeys were subjected to such an experiment.
They were infected twice, once to stimulate an immune response and a second time three weeks later. These very limited experiments showed that they did not develop symptoms again after such rapid reinfection.
If I have antibodies, am I immune?
There is no guarantee about this, which is why the World Health Organization (WHO) has expressed concern about countries that are adopting so-called "immunity passports" as a way out of lockdown.
The idea of the strategy is that if you pass the antibody test you will be safe to go back to work. This could be particularly valuable for health teams at home or in hospitals that come into contact with people in the group at risk of developing severe symptoms.
But while you find some antibodies in almost all patients, not all are the same. Neutralizing antibodies are those that adhere to coronavirus and are able to prevent them from infecting other cells.
A study of 175 patients recovered in China showed that 30% had very low levels of neutralizing antibodies.
That's why who says that "cellular immunity [the other part of the adaptive response] can also be critical to recovery."
Another issue is that just because you may be protected by your antibodies doesn't mean you can't still harbor the virus and transmit it to others.
Why is immunity important?
The issue of immunity matters for obvious health reasons and why it defines whether you will pick up covid-19 several times, and how often.
Immunity will also help define how deadly the virus is.
Understanding immunity could help relax confinement if it becomes clear who is not at risk of catching or spreading the virus.
If it is too difficult to produce long-term immunity, it may mean that getting a vaccine will be a more difficult task. Or it can change the way the vaccine needs to be used - it could be once in a lifetime or once a year, like the flu vaccine.
And the duration of immunity, whether by infection or vaccination, will tell us whether or not we are able to stop the virus from spreading.
All are big questions for which there is still no answer.