By Julian Tang
All viruses mutate. They do this to adapt and survive better in their specific host. The virus that causes Covid-19 is no different: it has moved from the animal realm, where it most likely originated in bats, to the human world. Since then, scientists have been locked in a battle between the spread of the virus and the ability to immunize against it. We now have the vaccines to protect us against Covid-19 – but what happens when this virus mutates further, as it likely will?
As lockdown restrictions ease, south London has already seen a cluster of new cases related to the South African variant. Over the next six months, dealing with emerging variants will be one of the major challenges that scientists face.
Some vaccines show promising signs of coping with new variants – the mRNA vaccines manufactured by Pfizer and Moderna seem to offer some protection against the variants first identified in Kent and South Africa. Most virologists think that Covid-19 vaccines will protect against severe disease and death, even in people who have been infected by a mutated strain of the virus.
But there are still potential difficulties. Being vaccinated doesn’t necessarily prevent you from becoming asymptomatically infected with Covid-19 and passing the virus on. Moreover, some vaccines may offer little or no protection against people becoming infected by a Covid-19 variant and passing this on to others (but experts believe that vaccines, together with a host’s natural immune response, should still offer enough residual protection to prevent severe disease and death). For example, clinical trials and laboratory studies show the AstraZeneca vaccine is only about 10% effective at protecting against the South African variant, but scientists still think the jab will protect against serious disease from this variant. Another question is how long vaccine immunity lasts for, and whether people become more susceptible to different variants as their antibody levels drop over time.
Virologists are hoping that the vaccines will induce what we refer to as long-term anamnestic immunity. Essentially, this would mean that after being vaccinated, a person’s B cells and T cells would remember (possibly for life) the particular S proteins of the virus to which they have been exposed. Therefore, if the person encountered the virus again, their immune system could rapidly respond by producing activated B cells (which produce antibodies) and T cells (which kill virus-infected cells) to fight it.
The virus that causes Covid-19 can produce new variants in at least two different ways. Firstly, the virus can undergo recombination, which is what happens when different pieces of genes from different viruses infect the same cell and mix to produce new variants.
Secondly, the virus can infect one immunocompromised patient for a long time, and then evolve within that patient to produce a new variant. This evolutionary process can lead to the emergence of a new variant that can evade the immune responses of its host, which may give it certain advantages when it spreads to other people.
So how can we combat new variants when they arise? Some of the first-generation Covid-19 vaccines, such as the mRNA ones, induce a wider range of protection than others. And there are already plans in place to update the Pfizer, Moderna and AstraZeneca vaccines to match the South African variant more closely. But it’s practically and economically difficult to continually update every Covid-19 vaccine each time a new variant appears.
Although vaccine redesign has been made easier by the introduction of “vaccine platforms”, there is still a significant time lag involved with manufacturing new shots and administering them en masse to vulnerable populations.
One option is to treat Covid-19 as we do seasonal influenza. Every year, experts carefully select an updated flu jab based on what they think will become the dominant influenza strain. We could apply this principal to Covid vaccines to cover as many new variants as possible. This is the purpose of the World Health Organization’s network of laboratories in the USA, UK, China, Japan and Australia, which work together to identify emerging strains of the influenza virus.
A global Covid-19 surveillance network could operate in the same way. This would involve hospital diagnostic labs from around the world submitting clinical samples to labs that could then compare the different viral sequences from different hemispheres. A surveillance network would determine how many Covid vaccines would need to be designed, manufactured and distributed to target emerging variants. A new vaccine might be required annually or every few years, depending on how quickly variants emerge.
Of course there is always the possibility of a doomsday variant that escapes all existing vaccines and natural immunity. With influenza, this is a very familiar risk: experts call it a “pandemic” strain. In years to come, this may also be a risk for the virus that causes Covid-19. Only constant vigilance, monitoring and collaboration will help the world detect such a variant, and prevent a catastrophic pandemic from occurring again.
Julian Tang is a clinical virologist and honorary associate professor in the respiratory sciences department at the University of Leicester.
Credit: The Guardian