Why vaccines might not be able to eliminate Covid-19: QuickTake

Photographer: Akos Stiller / Bloomberg

The road to eliminating Covid-19 is long and paved with uncertainties. Many countries rely on vaccines to create sufficient immunity in their populations so that SARS-CoV-2 is not able to find people who are likely to infect, which slows down and eventually stops the transmission of the coronavirus. But even with the deployment of highly effective vaccines, immunization coverage might not reach this level – the so-called herd immunity threshold – soon. On the one hand, it is does not know what level of immunity is required and whether vaccines will be powerful enough to do it. There is also the threat of emerging variants of coronavirus that may weaken the effectiveness of immunizations.

1. Can Covid-19 be eradicated?

No, so far only one human disease – smallpox – has been officially eradicated; that is, reduced to zero cases and stay there long-term without continued intervention measures. Smallpox was eradicated thanks to a very effective vaccine and the fact that humans are the only mammals to naturally susceptible to infection with smallpox virus which causes disfiguring, sometimes fatal disease. Humans are the only known reservoir of poliovirus, but it is still spreading a few countries causing crippling diseases despite widespread use of effective vaccines and a 32-year-old global eradication effort. SARS-CoV-2 is believed to persist in horseshoe-shaped nature bats, and is known to infect mink, cats, gorillas and other animals. To eliminate the virus, it would have to be banned from all susceptible species, which is not feasible. In countries that have succeeded in suppressing cases of Covid-19, disease elimination has been proposed instead.

2. What is elimination?

It was when efforts to suppress an epidemic resulted in zero new cases of disease or infection in a defined area over an extended period. There is no official definition of how long this should last. One proposal is to do it 28 days, corresponding to twice as long as the outer range of the SARS-CoV-2 incubation period – the time between infection and symptom onset. Some countries, like New Zealand, have not recorded any new cases for extended periods of time thanks to border closures, lockdowns and the diligent detection and isolation of cases. during a pandemic, which is an epidemic of a new infection across continents, supporting the elimination of any infectious disease nationwide is difficult, if not impossible, due to the threat of reintroduction of the virus into the country by infected international travelers.

3. Will the vaccines eliminate Covid-19?

It’s hard to say. We don’t know what a proportion of the population must be immune to keep the coronavirus from circulating, or if even the strongest vaccines will be able to stop it from spreading. One study estimated that to stop transmission, 55% to 82% of the population would need to be immunized, which can be achieved either by recovering from infection or by vaccination. However, collective immunity was not reached in Manaus, the capital of the state of Amazonas in Brazil, even after around 76% of the population infected. Still, there is reason to believe that mass inoculations will have a more potent effect, as vaccines appear to elicit stronger and longer lasting protection than a previous infection.

4. How effective will the vaccines be?

There is good evidence that the shots made by Pfizer Inc.-BioNTech SE and Moderna Inc. is very effective – up to 95% – at preventing recipients from developing Covid-19 on its own. However, no data has been published on their ability to prevent people from developing asymptomatic infections or passing the virus on to others. The gold standard in vaccinology is to stop infection as well as disease by providing what is called sterilizing immunity. But this is not always realized. The measles vaccine, for example, prevents infection so that vaccinated people do not spread the virus, while the vaccine for whooping cough protects well against serious disease, but is less effective at stopping infection. Encouragingly, a The study of Moderna’s Covid vaccine in monkeys suggested that it would reduce, if not completely prevent, transmission of the virus. Clinical trials using AstraZeneca Plc vaccine says it may be less than 60% effective in stopping infections – making herd immunity unlikely even if everyone in a population has received two doses.

5. How do virus variants come into play?

Researchers investigated the ability of antibodies in the blood of recovered Covid-19 patients to block the new, rapidly spreading B.1.1.7, 501Y.V2 and P.1 variants first reported in the UK , South Africa and Brazil. Some research has indicated that these strains may escape the immune protection offered by natural infection. Scientists have warned that laboratory studies are only indicative and there is no evidence that this is actually happening in the community, or that the antibodies generated by the vaccine will be less effective against the new strain.

6. Should Covid-19 vaccines prevent infection to stop cases?