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July 14, 2020

Herd immunity hope diminishing under new studies

Top Story

July 14, 2020

LONDON: Can you catch the coronavirus a second time? That remains unclear. A key question is whether antibodies produced by the body following an infection with the coronavirus provide some level of immunity, and if so, for how long, foreign media reported.

But we do have some clues. “We know from ‘normal’ coronavirus studies done in the past you can infect people after about a year following an initial infection,” said Dr Ben Killingley, consultant in acute medicine and infectious diseases at University College London hospital.

Dr Joshua Schiffer, an expert in infectious diseases at Fred Hutchinson Cancer Research Center in the US, added that any signs of reinfection would require detailed scrutiny. A new study of people who have caught and recovered from coronavirus raises the prospect that immunity to the virus may be short-lived. Scientists at King's College London studied how the body naturally fights off the virus by making antibodies, and how long these last in the weeks and months after recovery.

Almost all of the 96 people in the study had detectable antibodies that could neutralise and stop coronavirus. But levels began to wane over thethree months of the study. Our immune system is the body's defence against infection and it comes in two parts.

The first is always ready to go and leaps into action as soon as any foreign invader is detected in the body. It is known as the innate immune response and includes the release of chemicals that cause inflammation and white blood cells that can destroy infected cells. But this system is not specific to coronavirus. It will not learn and it will not give you immunity to the coronavirus.

Instead you need the adaptive immune response. This includes cells that produce targeted antibodies that can stick to the virus in order to stop it and T cells that can attack just the cells infected with the virus, called the cellular response.

This takes time - studies suggest it takes around 10 days to start making antibodies that can target the coronavirus and the sickest patients develop the strongest immune response.

If the adaptive immune response is powerful enough, then it could leave a lasting memory of the infection that will give protection in the future. It is not known if people who have only mild symptoms, or none at all, will develop a sufficient adaptive immune response.

Understanding of the role of T-cells is still developing. But a recent study found people testing negative for coronavirus antibodies may still have some immunity. For every person testing positive for antibodies, it was found two had specific T-cells which identify and destroy infected cells.

The immune system's memory is rather like our own - it remembers some infections clearly, but has a habit of forgetting others. Measles is highly memorable - one bout should give life-long immunity (as the weakened version in the MMR vaccine does). However, there are many others that are pretty forgettable. Children can get RSV (respiratory syncytial virus) multiple times in the same winter.

The new coronavirus, Sars-CoV-2, has not been around long enough to know how long immunity lasts, but there are six other human coronaviruses that can give a clue. Four produce the symptoms of the common cold and immunity is short-lived. Studies showed some patients could be re-infected within a year.

But the common cold is generally mild. There are two more troublesome coronaviruses - the ones that cause Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS) - in which antibodies have been detected a few years later.

"The question is not whether you become immune, it's how long for," said Paul Hunter, a professor in medicine at the University of East Anglia. He added: "It almost certainly will not last for life.

"Based on antibody studies in SARS it is possible that immunity will only last about one to two years, though this is not yet known for certain." However, even if you are not completely immune it is possible a second infection would not be as severe.

There were early reports of people appearing to have multiple coronavirus infections in a short space of time. But the scientific consensus is that testing was the issue, with patients being incorrectly told they were free of the virus.

Nobody has been deliberately reinfected with the virus to test immunity, but a pair of rhesus macaque monkeys have. They were infected twice, once to build up an immune response and then a second time three weeks later. Those very limited experiments showed they did not develop symptoms again after such a quick reinfection.

This is not guaranteed and that is why the World Health Organisation is nervous about countries using immunity passports as a way out of lockdown. The idea is if you pass the antibody test then you are safe to go back to work. This would be particularly valuable for staff in care homes or hospitals who come into contact with those at risk of developing severe symptoms.

But while you will find some antibodies in nearly every patient, not all are equal. Neutralising antibodies are the ones that stick to the coronavirus and are able to stop it infecting other cells. A study of 175 recovered patients in China showed 30% had very low levels of these neutralising antibodies.

That is why the WHO says "that cellular immunity [the other part of the adaptive response] may also be critical for recovery". Another issue is that just because you might be protected by your antibodies, it doesn't mean you cannot still harbour the virus and pass it onto others.

It matters for obvious personal health reasons and whether you will get Covid-19 multiple times and how often. Immunity will also affect how deadly the virus is. If people retain some, even imperfect, protection then it will make the disease less dangerous.

Understanding immunity could help ease lockdown if it is clear who is not at risk of catching or spreading the virus. If it is very difficult to produce long-term immunity, then it could make a vaccine harder to develop. Or it may change how the vaccine needs to be used - will it be a once a lifetime or once a year like the flu shot.

And the duration of immunity, whether by infection or immunisation, will tell us how likely we are to be able to stop the virus spreading. Experts say it is unlikely that the coronavirus can hide in the body and remain dormant for a long time. “It’s a term that’s borrowed from other viruses, especially the herpes virus family, that can hide in the body in a latent state to reactivate years later. [There is] no evidence of that at all for coronaviruses,” says a virologist.

However, a latest South Korean study found that the patients who test positive for the coronavirus weeks after recovering from COVID-19 probably aren’t capable of transmitting the infection, research from South Korea shows.

Scientists from the Korean Centers for Disease Control and Prevention studied 285 Covid-19 survivors who had tested positive for the coronavirus after their illness had apparently resolved, as indicated by a previous negative test result. The so-called re-positive patients weren’t found to have spread any lingering infection, and virus samples collected from them couldn’t be grown in culture, indicating the patients were shedding non-infectious or dead virus particles.

The findings, reported late Monday, are a positive sign for regions looking to open up as more patients recover from the pandemic that has sickened at least 4.8 million people. The emerging evidence from South Korea suggests those who have recovered from Covid-19 present no risk of spreading the coronavirus when physical distancing measures are relaxed.

On the other hand, an article in Vox quotes a medic saying that COVID-19 may also be much worse the second time around. “During his first infection, my patient experienced a mild cough and sore throat. His second infection, in contrast, was marked by a high fever, shortness of breath, and hypoxia, resulting in multiple trips to the hospital,” says the doctor.

Recent reports and conversations with physician colleagues suggest his patient was not alone. Two patients in New Jersey, for instance, appeared to have contracted COVID-19 a second time almost two months after fully recovering from their first infection. Daniel Griffin, a physician and researcher at Columbia University in New York, recently described a case of presumed reinfection on the This Week in Virology podcast.

“Also troubling is that my patient’s case, and others like his, may dim the hope for natural herd immunity. Herd immunity depends on the theory that our immune systems, once exposed to a pathogen, will collectively protect us as a community from reinfection and further spread,” say the medic. There are several pathways out of this pandemic, including safe, effective, and available therapeutics and vaccines, as well as herd immunity (or some combination thereof).