A single dose of the Pfizer-BioNTech vaccine protects against two of the most concerning coronavirus variants, but perhaps only in people who have overcome the infection naturally, research suggests.
An effective immunisation programme has long been hailed as a route out of the pandemic, however, the emergence of new variants in Kent, South Africa and India has left many concerned the virus may no longer respond to the UK's three approved jabs.
With most confident the vaccines will be at least somewhat effective, scientists from Imperial College London analysed the immune response of healthcare workers at London's Barts and Royal Free hospitals after one Pfizer-BioNTech dose.
Results suggest the workers who had overcome a mild or asymptomatic infection with the original coronavirus variant experienced "significantly enhanced protection" against the so-called Kent and South Africa variants post-jab.
The workers who had not fought off the coronavirus had a weaker immune response after the vaccine, potentially leaving them at risk of the variants.
A person's immune system may be "primed" after overcoming the coronavirus naturally, raising the potency of its response following the first vaccine dose.
The results may highlight the importance of getting the second jab when called up, with the first dose similarly priming the immune system.
"Our findings show people who have had their first dose of vaccine, and who have not previously been infected with SARS-CoV-2 [the coronavirus], are not fully protected against the circulating variants of concern," said lead author Professor Rosemary Boyton.
"This study highlights the importance of getting second doses of the vaccine rolled out to protect the population."
The Pfizer-BioNTech vaccine was approved after a late-stage trial demonstrated it was 95% effective at warding off severe disease with the variant that emerged in the Chinese city Wuhan at the end of 2019.
The trial participants received the two-dose regimen with a three-week interval between the jabs.
To maximise the number of people receiving a first vaccine, the UK extended the dose interval to up to 12 weeks. Many experts are optimistic this improves the immune response by giving it more time to prime.
Following the Pfizer-BioNTech vaccine's approval, the so-called B.1.1.7 coronavirus variant emerged in Kent in December 2020. The same month, the B.1.351 variant was identified in South Africa, both of which are considered "variants of concern".
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After analysing the healthcare workers' blood samples, the Imperial scientists found those who were known to have overcome the coronavirus naturally had significantly higher antibody, T cell and B cell counts following a single Pfizer-BioNTech dose.
Antibodies are proteins that are specific to a pathogen, like the coronavirus. They lock onto the virus' surface, neutralising or "marking" it for destruction by other immune cells.
So-called "helper T cells" stimulate antibody production and assist in the development of "killer T cells", which directly destroy body cells that have already been infected.
T cells also send out messages instructing the rest of the immune system, like infection-fighting B cells, to ramp up its response.
"Interestingly", mutations in the Kent and South Africa variants resulted in reduced, enhanced or unchanged T cell immunity between the previously-infected workers after a jab, depending on their genetics.
The extent of protection given by T cells is unclear.
"Our data show natural infection alone may not provide sufficient immunity against the variants," said Professor Boyton. "Boosting with a single vaccine dose in people with prior infection probably does.
"As new variants continue to emerge, it is important to fast track global rollout of vaccines to reduce transmission of the virus and remove the opportunities for new variants to arise."
Variants come about when the coronavirus acquires mutations that may change its action, for example genetic modifications that enable the virus to transmit more easily.
Mutations occur when the virus replicates, which takes place as it spreads from person to person.
The scientists did not look at other variants, like those that are thought to have emerged in Brazil and India. Nevertheless, similar results may apply, they added.
It is unclear if the UK's other two approved vaccines – University of Oxford-AstraZeneca and Moderna – have the same effect.
"At a time of generally improving outlook in those countries with substantial vaccine rollout programmes, this study reminds us of the need to be vigilant about the threat of the variants," said co-author Professor Danny Altmann.
"Most vaccinated people in the UK have received just one dose.
"While we know this offers remarkable protection against the original virus, our data suggest this leaves people susceptible to variants of concern."
More than 34 million first doses of any of the UK's three vaccines have been administered in the UK, compared to 14 million second doses.
"Our study offers reassurance and a warning," added co-author Professor Áine McKnight, from Queen Mary University.
"We show current vaccines offer some protection against variants of concern, however, people who have received only the first course of a double dose vaccine show a more muted immune response.
"We must ensure the global vaccination programme is fully implemented.
"Current events in India make painfully clear the cost of complacency."
India has reported more than 18 million confirmed coronavirus cases since the outbreak emerged, second only to the US. Its death toll recently doubled from 200,000 to more than 400,000.
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