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Some human antibodies target the spike protein on the surface of the coronavirus. Photo: EPA-EFE

Coronavirus immunity: how can we develop it and how long might it last?

  • While the world uses emergency measures to slow transmission, stopping it also rests on the human immune system’s ability to respond
  • Recovered patients’ immunity expected to last longer than that for influenza, raising hopes for vaccines to have long-term effectiveness
Developing immunity to the coronavirus is key to daily lives returning to normal. Since the Sars-CoV-2 virus – which causes the disease Covid-19 – has been known for only a short period of time, there is much to be studied regarding humans’ immune response to it.

The South China Morning Post spoke to researchers about what we know about immunity so far.

How is immunity to the virus developed? 

Ashley St John, an immunologist from Duke and National University of Singapore’s medical school, said that most of our information on immunity to Sars-CoV-2 has been borrowed from the severe acute respiratory syndrome (Sars) and Middle East respiratory syndrome (Mers) outbreaks, and that further studies were needed to verify whether it is correct.

Infection with Sars or Mers results in a protective immune response in which T cells and antibodies specific to the pathogen are generated. T cells are important for killing virus-infected cells, while antibodies bind to the virus and prevent it infecting new cells, according to St John.

“Pre-clinical vaccine studies for Sars and Mers have shown that ‘mucosal immunity’ is especially critical for protection against coronaviruses,” she said. “This kind of immunity gives you protection in your lungs, gut and nasal mucosal surfaces.”

What is herd immunity? And can it stop the coronavirus?

Vaccines can generate immunity to the virus without people becoming sick. Researchers are working to produce a vaccine that simulates the immune responses of healthy individuals who cleared the virus, St John said.

For Sars-CoV-2, those who recover develop antibodies that can neutralise the virus. Some of the antibodies target the spike protein on the surface of the virus, which is involved in the virus binding to cells to allow infection, she said.

This is good news for the development of vaccines and long-term immunity, according to virology professor Malik Peiris from Hong Kong University.

“The protective immunity to this virus, as well as Mers, is targeted at exactly the site where the virus binds to the cell,” he said. “This means the virus cannot afford to mutate that site.”

The mutation rate of the new coronavirus is also not as high as for other RNA viruses (those that have ribonucleic acid as their genetic material), such as influenza or HIV. Therefore there is a very high likelihood that vaccines for this coronavirus would have long-term effectiveness, Peiris said.

How can we find out whether we are immune?

Serologic tests, or immunity tests, look for antibodies or proteins produced by humans’ immune system to attack pathogens in the blood. Running wide-scale testing would provide a more accurate picture of how many people have been infected and gained immunity to the virus, according to Peiris.

This is important given that Peiris described the number of people who have been infected as an “iceberg”, and serologic tests would help to reveal some of the hidden cases.

“This iceberg, we don’t know how big this is,” he said. “The only way to find out the full size of the iceberg is to do serologic testing.”

Testing could also help in places like mainland China, where the focus has turned to gradually reopening businesses and schools. On Thursday, 91 new cases were reported on the mainland, of which 60 were asymptomatic.

If a large proportion of the population has been infected, then it should be safe to resume normal activities faster, Peiris said.

“But then you have to be much more careful because the disease can come back and we know there are imported cases. So for these decisions to be made in the right way, antibody testing is extremely important.”

Is it possible to be reinfected?

Reports of patients who were discharged from hospital having tested negative for the coronavirus before later testing positive again have raised fears of reinfection.

Quarantine facilities in Wuhan, the outbreak’s initial epicentre in central China, have reported that about 5 to 10 per cent of their recovered patients tested positive again, according to health newspaper Life Times. Health officials in southern China’s Guangdong province said about 14 per cent of those discharged from hospital later tested positive again.

However, this was probably because of the complexities in collecting samples rather than about immunity, according to Leo Poon Lit-man, the head of public health laboratory sciences at Hong Kong University.

Coronavirus treatments and vaccine trials in the works

“It is unlikely a patient will become reinfected again in such a short amount of time – days or weeks,” Poon said. “That is because the patient will have enough antibodies in the blood to suppress the reintroduction of the virus.”

When a patient recovers from Covid-19, the amount of the virus in their cells decreases and is most likely not to be infectious towards the end of their illness, according to Poon. However, the polymerase chain reaction (PCR) test that detects the RNA of viruses is very sensitive and able to detect even small traces of the virus.

“Maybe occasionally you have a recovering patient who has tested negative a few times, then is suddenly positive. It could happen, but they are certainly not infectious,” he said.

How long does immunity to Sars-CoV-2 last?

It is not yet known how long immunity to such a newly discovered virus as Sars-CoV-2 would last, but it is hoped that it would be long-term, Peiris said.

“It seems almost all of the patients develop antibodies,” Peiris said. “We haven’t followed them up for long enough to know how long it lasts. One would hope that it would last for some period, but we don’t know the answer to that at the moment.”

Why China’s military scientists want to test coronavirus vaccine abroad

A study by China’s Centre for Disease Control and Prevention (CDC), published in the Antiviral Research journal in 2017, said Sars antibodies were detected in most patients two years after infection. Meanwhile, T cells, the other immune response, were found in patients more than 10 years after Sars infection.

Mers antibodies were present in people about 12 to 13 months after infection, according to studies in Jordan and Saudi Arabia. Based on the findings of those, Chinese CDC researchers said antibody levels declined more rapidly in Mers-CoV patients than those in Sars survivors.

Research done by Peiris and his partners in South Korea found that some Mers patients, particularly mildly infected cases, did not show antibody responses, but those who did had those antibodies for at least a year.

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