Photographer: Akos Stiller / Bloomberg
Photographer: Akos Stiller / Bloomberg
The road to eliminating Covid-19 is long and full of uncertainties. Many countries are counting on vaccines to build sufficient immunity in their populations so that SARS-CoV-2 is unable to find people susceptible to infection, causing the transmission of the coronavirus to slow and eventually stop. But even with the launch of highly effective vaccines, immunization coverage may not reach that level – the so-called herd immunity threshold – coming soon. On the one hand, it is it is not known what level of immunity is needed and whether vaccines will be powerful enough to achieve it. There is also a threat of emerging variants of coronavirus that can weaken the effectiveness of immunizations.
1. Can Covid-19 be eradicated?
No. So far, only one human disease – smallpox – was officially eradicated; that is, reduced to zero cases and maintained there for a long term without continuous intervention measures. Smallpox has been eradicated thanks to a highly effective vaccine and the fact that humans are the only mammals that are naturally susceptible to infection with smallpox virus that causes disfiguring, sometimes deadly, disease. Humans are the only known reservoir of poliovirus, but it still spreads across some countries, causing a paralyzing disease despite the widespread use of effective immunizations and a 32-year-old patient global eradication effort. SARS-CoV-2 is believed to persist in horseshoe nature bats and is known to infect mink, cats, gorillas and other animals. Eliminating the virus would require banning it from all susceptible species, which is not feasible. In countries that have successfully suppressed the Covid-19 cases, elimination of the disease was proposed instead.
2. What is elimination?
That’s when efforts to suppress an outbreak resulted in zero new cases of disease or infection in a defined area over a sustained period. There is no official definition of how long it should last. One proposal is to make 28 days, corresponding to twice as much as the external variation of the SARS-CoV-2 incubation period – the time between infection and the onset of symptoms. Some countries, such as New Zealand, have reached zero new cases for long periods using border closures, blockades and diligent detection and case isolation. During a pandemic, which is an outbreak of a new infection on continents, supporting the elimination of any infectious disease across the country is challenging, if not impossible, because of the threat of the virus re-entering the country of infected international travelers.
3. Will vaccines eliminate Covid-19?
It is hard to say. It is not known what proportion of the population must have immunity to prevent coronavirus circulation, or whether even the most potent vaccines will be able to prevent its spread. One study estimated that, to stop transmission, 55% to 82% of the population would need to have immunity, which can be achieved by recovering from an infection or by vaccination. However, the herd’s immunity has not been reached in Manaus, the capital of the state of Amazonas in Brazil, even after about 76% of the population has been infected. Still, there are reasons to believe that mass inoculations will have a more powerful effect because vaccines appear to produce stronger and more durable protection than a previous infection.
4. How effective will vaccines be?
There is good evidence that shots fired by Pfizer Inc.-BioNTech SE and Moderna Inc. is very effective – up to 95% – in preventing recipients from developing Covid-19 on their own. However, no data has been released on its ability to prevent people from developing asymptomatic infections or transmitting the virus to others. The gold standard in vaccinology is to stop infection as well as disease by providing so-called sterilizing immunity. But it is not always achieved. The measles vaccine, for example, prevents infection so that vaccinated people do not spread the virus, while the vaccine against Whooping cough protects well against serious illnesses, but is less effective at preventing infection. Encouragingly, a Modern Covid vaccine study in monkeys has suggested that it will reduce, if not completely prevent, virus transmission. Clinical trials using The AstraZeneca Plc vaccine indicates that it may be less than 60% effective in preventing infections – making it unlikely to achieve collective immunity, even if everyone in a population receives two doses.
5. How do virus variants influence?
The researchers studied the ability of antibodies in the blood of patients recovered from Covid-19 to block the new B.1.1.7, 501Y.V2 and P.1 variants of rapid spread reported for the first time in the United Kingdom, South Africa and Brazil . Some research has indicated the potential of these strains to escape the immune protection provided by natural infection. The scientists warned that the laboratory studies are only indicative and there is no evidence that this is actually occurring 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 contain cases?
No. Vaccines it doesn’t have to be perfect to have a public health benefit. New Zealand vaccinologist Helen Petousis-Harris points to rotavirus and chickenpox as examples of diseases that have been “virtually eliminated with vaccines that are very good at preventing serious diseases, very good at preventing any disease, but which do not completely prevent infection at all.” Because SARS-CoV-2 spreads through respiratory particles in the throat and nose of an infected person, a vaccine that reduces the amount of virus in the respiratory tract or reduces the frequency of an infected person’s cough can decrease the likelihood of transmission for other people and decrease the number of effective reproduction (Re), which is the average number of new infections estimated as resulting from a single case. Mike Ryan, head of The World Health Organization’s emergency program, told reporters on January 25 that, instead of focusing on eliminating SARS-CoV-2, success should be seen as “reducing this virus’s ability to kill, to put people at the hospital, to destroy our economic and social lives. “
Explosion to Elimination
A model of how this can work
Source: Los Alamos National Laboratory (Wuhan R0 and serial range)
7. What if Covid-19 is not eliminated?
David Heymann, President of the WHO Strategic and Technical Advisory Group on Infectious Risks, warned at the end of 2020, “it seems that the fate of SARS-CoV-2 is to become endemic”. Endemic viruses circulate continuously in the community, often causing periodic spikes when disease characteristics and patterns of human behavior favor transmission. Examples include norovirus, the notorious cause of gastroenteritis on cruise ships and a myriad of viruses, including four coronavirus, which cause the common cold, especially during the winter.
8. What can be the implications?
It is not known how things will evolve, but researchers have started to create scenarios. People who survived Covid-19 and those vaccinated against it are likely to be protected against the disease for some time. It is likely that reexposure to the virus or a booster injection of the vaccine will strengthen your protection. As more and more people develop immunity in this way, the virus will find those who are not yet immune, as long as collective immunity is not established to protect them. This means that people who cannot be vaccinated – because their immune systems are compromised, are either allergic to the ingredients of the vaccine or are very young (none of the vaccines authorized in Western countries have been approved for children) – will remain vulnerable. Some scientists have predicted that once the endemic phase is reached and the primary exposure to the virus is in childhood, SARS-CoV-2 may be no more virulent than the common cold.
The Reference Shelf
- Devi Sridhar and Deepti Gurdasani from the University of Edinburgh detail the difficult lessons learned from a largely uncontrolled epidemic of Covid-19 in Manaus, Brazil.
- Anita Heywood and Raina MacIntyre of the University of New South Wales explain the eradication, elimination and suppression of diseases and which elimination of Covid-19 it would seem. MacIntyre also features principles of vaccine programs to control Covid-19.
- Related QuickTakes on how to vaccine hesitation threatens to delay the end of the pandemic, why delaying the second injection of a Covid-19 vaccine is confusing because the mutated variants are just as worrying, as the coronavirus is transmitted, the vaccine launch if you can be forced to be vaccinated, coronavirus treatments and the unanswered questions about the virus.
– With the help of Alisa Odenheimer