We are nearing two years of the first reported cases of Covid-19 to come out of China. It is natural to wonder about the future trajectory of the pandemic. As I wrote in COVID-19: Separating Fact from Fiction, the pandemic will indeed end. All pandemics end.
The World Health Organization declared the pandemic in March 2020. And it will be the organisation that declares when the pandemic officially ends. But the end of pandemic will not mean that Sars-CoV-2 will disappear. There are simply too many people infected in too many countries, too many rapidly emerging variants with a high degree of infectiousness, and too many animals which can act as reservoirs for eradication to be viable outcome.
The pandemic will not end everywhere all at once. What the world can expect is a degree of endemicity, in which there will be localised outbreaks in various parts of the world. And there may be spikes exacerbated by seasonal changes and behaviour.
There are many factors that determine when and how the pandemic ends, but I would put them into two categories — viral evolution and immunity. Our clearest path out of the pandemic remains mass vaccination, just as it was at the start of the pandemic. But the infectiousness of variants that have arisen in the past year has changed the equation — nearly everyone will need to be infected or vaccinated for this crisis to be behind us.
A year ago, most virologists did not predict the rapid emergence of variants of Sars-CoV-2. Right now, of the variants we know, the Delta variant (which was first detected in India) spreads most rapidly. Although other variants are emerging, Delta is still the most problematic because it replicates much better than the ancestral Wuhan lineage.
Is Delta as bad is it is going to get? In India, after the devastating wave that occurred from March through June, restrictions were also lifted. When the United Kingdom loosened restrictions in the midst of rising Delta infections in July, many public health officials criticised the decision; the subsequent explosion in infections that they predicted has not happened. Predictive models, charts, as well as the increased pace of vaccinations are cause for cautious optimism. China has vaccinated a billion people. Although India trails in the percentage of population fully vaccinated, it is also approaching a billion doses administered.
That said, new variants will emerge, and they may supplant Delta in the future. Where and when these variants will emerge is much harder to predict. We know that immunosuppressed individuals who received antibody therapy had persistent infections that lasted for months. In these people, viral lineages acquired many mutations and became variants. But variants can also emerge over time as a result of rapid spread through many people instead of in one person.
Vaccine inequity has been a problem in 2021. Parts of the world will still be unvaccinated at the end of 2022. Variants can emerge in any part of the world where the virus is spreading. As more people are vaccinated, there will be selection pressure on variants to escape from vaccine-induced immunity.
Which brings me to the second set of factors that will determine how quickly the pandemic ends — immunity against emerging variants. As a substantial portion of the population is vaccinated or become immune post-infection, peaks of successive waves of infection may subside.
But the speed by which children are vaccinated will remain a factor in transmissions in many countries. Older children have fared relatively well compared to other populations with respect to Covid-19 outcomes. However, millions of babies are born each year who are immunologically naïve. Antibodies from mothers who are vaccinated or have suffered infections may offer some protection.
The robustness of immunity after vaccination also matters. Populations that received both vaccine doses earlier this year are losing the ability to prevent transmissions caused by variants. With waning immunity and variants that are mismatched to existing vaccines, vulnerable populations will need booster shots to prevent severe outcomes. Here, a comparison of Covid-19 vaccines can be made to flu vaccines; annual flu shots protect well against severe disease, but not against infection from all circulating strains.
What do we care about when we talk about the pandemic? Infections and transmissions are important because they allow us to determine the parameters of the epidemic curve. The severity of illness is important since it determines the burden on hospitals and health care providers. The mental and physiological effects of post-Covid (also known as Long Covid) are important. But even now, the death toll is a reminder that we are still in a pandemic. Conversely, as the death toll declines and remains low, parts of the world will approach normalcy in a phased manner.
New drugs such as molnupiravir, which Merck claims reduces hospitalisation and death by 50%, are heading towards approval. Diagnostic tests are now widely available. Masks, ventilation, and air-filtration devices have become commonplace. But getting vaccinated is still the best anti-Covid measure. Despite some breakthrough infections and declining antibody amounts, approved Covid-19 vaccines have held up well in preventing severe disease and death.
There’s more to immunity than antibodies. And vaccines provide excellent cellular immunity to all known variants.
Many scientists think that emerging variants of Sars-CoV-2 will cause asymptomatic or mild illness in most people who have prior immunity (either due to infection or vaccination). Four human coronaviruses that are endemic to humans cause mild illness in most people. That is certainly the ideal scenario for Sars-CoV-2 endemicity.
There are historical reminders from earlier pandemics too. The H1N1 pandemic over a century ago was caused by a different respiratory virus. There is a misconception that the H1N1 influenza strain of 1918 disappeared in 1919 when the pandemic ended. It didn’t. As Jeffrey Taubenberger and David Morens noted in an article entitled “1918 Influenza: the Mother of All Pandemics”, most of the influenza cases (both seasonal and pandemic) that occurred in the past century were caused by descendants of the 1918 virus.
Nearly two years into the outbreak, there are still uncertainties that preclude putting a timeline on the end of the pandemic. But we know what remains to be done and what we should look out for. We will learn to live with Sars-CoV-2 variants in a post-pandemic world.
Anirban Mahapatra, a microbiologist by training, is the author of COVID-19: Separating Fact From Fiction
The views expressed are personal