Why do some people never seem to get Covid-19? Scientists may have the answer

Being exposed to other coronaviruses such as the common cold can give immunity

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Catching the common cold could help the body stave off Covid-19 and pave the way for a universal vaccine offering protection against the wider coronavirus family, research suggests.

The human body's ability to strengthen its immune response against the virus – be it through vaccination or infection – is key to the scientific world's efforts to support the global recovery from the pandemic.

It may be that abortive infections, where a person is exposed but never has enough virus in their system to test positive, may affect the immune system despite Sars-CoV-2, the virus that causes Covid-19, being rapidly cleared from the body.

There is also evidence previous exposure to other coronaviruses – such as a cold – may give individuals protection in the event they are infected with Sars-CoV-2.

We know with Covid, the more severely ill you get, the stronger the immunity after you’ve recovered - unless you die
Prof Paul Hunter, University of East Anglia

Here, The National takes a look at what the studies found and speaks to experts.

Why might brief infections boost protection?

It has long been known many Covid-19 infections are asymptomatic, and the easily-spread Omicron variant has a higher rate of such cases than other common variants.

In these instances, a person may have no symptoms, but a PCR test (which looks for viral genetic material) or a lateral flow test (which looks for viral antigens, which are types of proteins) comes up positive.

Abortive infections are a separate category, because the person has been exposed to the virus, but it has never replicated enough to be detected by PCR or lateral flow tests.

Dr Andrew Freedman, an infectious diseases expert at Cardiff University in the UK, said it was theoretically possible such abortive infections were strengthening the immunity of individuals.

“If they have good T cell immunity, although they’re getting exposed, they’re not developing a full-blown infection,” he said.

T cells are immune system cells that can help another form of immune system cell, B cells, to produce antibodies. T cells can also directly destroy human cells that are infected with a pathogen. T cells are often associated with long-lasting immunity.

Professor Paul Hunter, an infectious diseases specialist and professor in medicine at the University of East Anglia in the UK, said the idea abortive infections enhanced immune protection against Sars-CoV-2 was “plausible” but had yet to be proved.

However, in mild infections that don’t have much effect on a person’s T-cell count, they probably have “a small if any” effect on a person’s immunity.

“We know with Covid, the more severely ill you get, the stronger the immunity after you’ve recovered, unless you die,” Prof Hunter said.

Could the common cold help fight off the virus?

The ability to shed SARS-CoV-2 quickly may, according to Dr Freedman, be the result of a “different coronavirus infection with cross-reactive immunity”.

“Previous coronaviruses may provide some protection,” Dr Freedman said.

So being infected with a coronavirus that causes the common cold, for example, may give a person protection against the coronavirus that causes Covid-19.

Dr Leo Swadling, who studies T cell immunity at University College London (UCL), has been looking at this issue with colleagues, including Professor Mala Maini.

They and other researchers published a paper last month that discussed how some people who may have had a brief exposure to Sars-CoV-2 cleared the infection without testing positive or developing antibodies.

Their suggestion is that “pre-existing memory T cell responses”, resulting from infections with other coronaviruses, could be behind these abortive infections.

They looked at healthcare workers at St Bartholomew’s Hospital in London who tested negative for antibodies against Covid-19.

These individuals have T cells that recognise the first proteins a coronavirus makes after it enters a cell, which make up the virus’s replication-transcription complex (RTC).

These T cells were detected in blood taken in March 2020 and in a set of samples taken before the pandemic.

“These T cells likely came from previous exposure to endemic common colds coronavirus infections (Human coronaviruses, HCoV), although some pre-existing T cells also likely came from exposure to totally different infections,” Dr Swadling said.

Will this lead to a universal coronavirus vaccine?

The pre-existing T cells analysed in the studies target parts of the virus that are the same in HCoV and Sars-CoV-2. In lab tests, the researchers found these pre-existing T cells, and T cells generated by an abortive infection, recognise both Sars-CoV-2 and HCoV.

Exposure to Sars-CoV-2 that results in an abortive infection was found to increase the numbers of these T cells – suggesting it may strengthen a person’s immunity against the coronavirus.

However, Dr Swadling said it remains unclear if immunity is strengthened by abortive infections.

“But we know from many Sars-CoV-2 studies that there is now a strong association between having an early Sars-CoV-2 T cell response and being protected from severe disease, and that these pre-existing T cells are recruited into early immune responses,” he said.

“So the fact that T cells are important for protection from disease and that they are expanded by abortive infection suggests that they would help on re-exposure.”

The way in which exposure to some coronaviruses may confer immunity against others opens the possibility of developing vaccines that can protect against multiple coronaviruses, something scientists are looking at.

A vaccine that activated T cells to destroy human cells that are producing the RTC may be effective against multiple coronaviruses – including different variants of Sars-CoV-2, and even viruses circulating in animals that have yet to spill over into the human population.

Updated: February 23, 2022, 10:55 AM