SYDNEY — Scientists hope that a research breakthrough may provide the long-term immunity needed to develop an effective vaccine against Covid-19, as a second wave of the deadly virus spreads across the globe.
A study by the Doherty Institute in Melbourne, Australia, suggests that bone marrow and thymus cells (B and T cells) could offer more lasting protection than antibodies, which in Covid-19’s case usually disappear from the body in a matter of weeks.
Confirmed by laboratories in the United Kingdom, the research helps explain why some people get only mild symptoms from Covid-19, and others none at all.
“We found that those who showed strong neutralizing antibody activity had a robust B cell response, but most surprisingly, we also found that a particular subset of T cells, called T-follicular helper cells, was a great predictor of an effective immune response,” said Dr Jennifer Juno of the University of Melbourne, a postdoctoral researcher at the institute.
About two dozen potential vaccines have reached the clinical phase of trials on humans, with two — in China and the UK — now at phase three, where large-scale testing can begin. In the Asia-Pacific, eight Chinese trials are underway on humans, two each in India and Australia and one in Japan.
The race to find a vaccine has intensified as much of the world encounters secondary outbreaks, and evidence builds that the virus is mutating into more infectious strains that could complicate efforts to find a treatment.
Countries that had achieved success in containing the spread of Covid-19, including Hong Kong, South Korea, Australia, Spain, Germany, Iran and South Africa, have reimposed lockdowns after new clusters erupted.
There are now thought to be at least four separate strains of the virus, and some scientists believe there will be 20 by the end of the year. Nearly 15 million people have been infected worldwide, and some 600,000 have died.
Two data sets released today (July 22) by the Centers for Disease Control and Prevention in the US indicate the toll may be vastly underestimated, with the true number of US cases possibly ten times higher than reported. According to official figures, there have been almost 3.9 million US cases.
Vaccines under development by Oxford University in the UK, CanSino Biologics in China and Moderna in the US have induced good immune responses, but tests were limited in scale and it is not clear if they will give lasting protection.
Most vaccines rely on protection from antibodies, which are produced after white blood cells and chemicals identify a threat and grab onto virus particles to stop them infecting cells.
However, scientists at Kings College in London reported this week that levels of the antibodies in their test subjects fell after 20 days and some people had none at all in two months.
Only 17% of patients still had a potent level of antibodies after 57 days, which could explain why some people become reinfected with Covid-19. It may make vaccines impotent and lead to a permanent cycle of infections, as 60% of people need to be resistant at one time for so-called herd immunity.
T and B cells are the main cellular components of an adaptive immune response: B cells attack viruses outside the cells, while the T cells tackle infected cells before the virus has a chance to duplicate and spread.
“Looking at Covid-19 patients – but also I’m happy to say, looking at individuals who have been infected but did not need hospitalization – it’s absolutely clear that there are T cell responses,” said Adrian Hayday, an immunology professor at King’s College.
“And almost certainly this is very good news for those who are interested in vaccines because clearly we’re capable of making antibodies and making T cells that see the virus.”
On the downside, T cells aren’t that effective at reaching airways where the virus first arrives, and there is disturbing evidence that they become less numerous after the age of 30, which may be why elderly people are more vulnerable to Covid-19 and the young seem to have more immunity.
Researchers believe that 50% of people on average have a pre-existing resistance to the virus due to high levels of T cells that were possibly created during a previous infection, such as a bout of influenza.
People infected with Severe Acute Respiratory Syndrome (SARS), which is also a coronavirus, were found to still have T cells years after they recovered from the illness.
Even if T cells eventually die off, they could still play a valuable role in the development of vaccines if they can be engineered to provoke a response in other parts of the immune system that provide more lasting benefits.
“There really is an enormous spectrum of vaccine design,” said Hayday. “So if we can stop whatever it’s doing to the T cells of the patients we’ve had the privilege to work with, then we will be a lot further along in controlling the disease.”