In a cave in the Chinese province of Yunnan, creamy dollops of limestone are suspended from the ceiling. It’s warm, humid and stinky. Nestled between the rock formations are thousands of horseshoe bats, squabbling and jostling for roosts.
It was a cave like this that possibly was the source of the SARS-CoV-2 virus that devastated the world in 2020. While the vaccine response to SARS-CoV-2 was developed in record-breaking time, there is very little to suggest that we would be able to move so quickly against a new virus. And a new virus is just a matter of time.
It was bats in a cave in China that harbored SARS-CoV-1 before it jumped to humans in 2002, killing 800 people. Lyssa virus, Hendra virus, Marburg, Nipah and Ebola are all viruses that jumped from bats to humans. It is not entirely clear why bats have been associated with such spillovers. One possibility is that destruction of their habitats by deforestation or global warming have caused bats to migrate to human-dense areas, resulting in spillovers. Bats are also among the most populous mammals in the world, and despite their bad reputation, they play an important role in the ecosystem.
Bats are highly communal creatures, their massive roosts providing opportunities for viruses to spread and mutate. There's also evidence that bats have a robust first-line immune response, releasing proteins called interferons that interfere (hence the name) with viruses’ ability to replicate. Such a challenging environment for a virus may force mutation, increasing the risk it could evolve to be more virulent and able to bypass the body’s first line of defense, which is very similar among mammals.
When SARS-CoV-2 emerged in late 2019, the world had already done the groundwork for developing a vaccine. Scientists already knew the most vulnerable part of coronaviruses (the spike protein), based on prior studies with related coronaviruses, including SARS-CoV-1 and MERS. In addition, many of the vaccine platforms used in the battle against SARS-CoV-2, such as the mRNA or adenovirus, had already been tested for other diseases. It was therefore relatively easy to apply the knowledge to the new, similar SARS virus.
Another reason why the SARS-CoV-2 vaccine was developed so fast is because there was a large financial investment and high number of infections to test several candidate vaccines. It is easier to test a vaccine when there are millions of cases worldwide, allowing rigorous statistical comparison of efficacy between the vaccine and placebo. There was no shortage of volunteers willing to trial the new vaccine.
New viruses have emerged multiple times in history. About a century ago the Spanish flu pandemic killed millions of people, and various forms of flu continue killing people every year. HIV was first documented in central Africa in the 1950s, and perhaps made the leap as a result of humans encroaching on ape habitat. Some scientists also speculate that some of the viruses that cause the common cold were previous coronavirus pandemics that became endemic in the human population.
New viruses can be generated by mutation or recombination of existing viral components. New viral epidemics could arise from the animal world, as well as from humans. Every time a virus enters a new host and replicates, it incorporates errors or mutations, and over time, these changes can significantly alter the biology of a virus.
There are more than 20 known families of viruses. Only a fraction of these cause disease in humans. Highly effective vaccines exist against smallpox virus, polio virus, yellow fever virus, measles/mumps/rubella, hepatitis B virus, papillomavirus, rotavirus, rabies virus; and some partially effective vaccines, including those against influenza.
While the world developed the SARS-CoV-2 virus successfully we may not be as prepared for the next virus. In the case of HIV, we still do not have an effective vaccine after almost 40 years. Other viruses, such as influenza, have high rates of mutation, substantially more than that of coronaviruses, creating an obstacle for developing a vaccine.
But the world can learn from the lessons of COVID. Data shows that even a vaccine that is designed for a viral relative could confer partial protection against disease. Mice vaccinated with an old SARS-CoV-1 vaccine from 2003, were somewhat protected against the SARS-CoV-2 virus. This means that the vaccine does not have to be 100 percent matched.
By knowing the viral sequences that are circulating in bats and other animal hosts, we may be better prepared for a future pandemic, because it would be possible to pre-emptively understand the biology of the virus and develop vaccines.
The world could potentially make a stockpile of vaccines based on the known bat coronavirus sequences, and during a future pandemic test the vaccine that is most genetically similar in a small number of people, as a quick stopgap until more ideal vaccines are developed.
Such work would require a substantial investment in basic science: sampling bats, sequencing and categorizing their viruses, building a library of vaccines based on these known viruses. This effort would need to be continuous, as viruses continue to evolve.
The technology exists. The only question is whether the world is prepared to invest more in a basic science insurance policy for a future pandemic.
(Pablo Penaloza-MacMaster, is an assistant professor at Northwestern University, USA.)
