In Part I of this series, we discussed a recent poll taken of over 100 scientists working on the Covid-19 coronavirus if they thought it would be entirely eradicated through the widespread dissemination of vaccines or if it would become endemic, continuing to circulate in pockets of the global population. In this poll, 89% of the scientists believed it would become endemic.
The Nature article that reported this poll went on to discuss possible future scenarios. One scenario posits that children will contract the Covid-19 coronavirus like they contract the common cold, another coronavirus, today. The symptoms will be mild, like the common cold, and will not require any significant lifestyle changes. This will come about through widespread vaccination and herd immunity.
This scenario would follow what we have already seen in four common coronaviruses known as OC43, 229E, NL63 and HKU1. At least three of these four endemic coronaviruses have been circulating in human populations for hundreds of years. Two of them are responsible for approximately 15% of respiratory infections.
Jennie Lavine, an infectious disease researcher at Emory University, Atlanta, says the virus would become a foe first encountered in early childhood, when it typically causes mild infection or none at all. Lavine and her colleagues developed a model that show how most children first come down with these viruses before the age of six and develop immunity to them. That defense wanes pretty quickly so it is not sufficient to block reinfection entirely, but it seems to protect adults from getting sick.
It is unknown if the SARS-CoV-2 (Covid-19) virus will behave in the same way. One large study of people who have been infected with Covid-19 suggests that their levels of neutralizing antibodies start to decline after 6 – 8 months. But their bodies also make memory B cells, which can manufacture antibodies if a new infection arises, and T cells that can eliminate virus-infected cells.
Daniella Weiskopf, an immunologist at La Jolla Institute for Immunology, who co-authored the study, says if most people develop life-long immunity to the virus, either through natural infection or vaccination, then the virus is unlikely to become endemic. But immunity might wane after a year of two – or the virus evolves to escape it by developing new variants. More than half the scientists in the Nature survey think waning immunity will be one of the main drivers of the virus becoming endemic.
Vaccines and Herd Immunity
We are already seeing a decline in the daily number of new infections since the vaccines have become available. The number of serious illnesses is also declining. But it will take longer to see how effectively vaccines can reduce transmission. Data from some clinical trials suggest that vaccines that prevent symptomatic infection might also stop people from passing on the virus.
If vaccines do block transmission, and remain effective against new variants, it might be possible to eliminate the virus in regions where enough people are vaccinated so that they can protect those who are not, contributing to herd immunity. It is estimated that a vaccine with 90% or greater efficacy will need to reach 55% of the population to achieve herd immunity as long as mitigating factors such as masks and social distancing remain in place. If these measures are eliminated, the vaccine would have to be given to almost 67% of the population.This will be a greater challenge in some countries than others.