Several recent outbreaks show that even a very high vaccination rate (over 90%) will not stop the spread of the infection. All indications are that SARS-CoV-2 will eventually become a recurrent seasonal virus. “It will be very difficult or even impossible to achieve herd immunity,” the scientists write.
The pandemic has raised a number of important questions that so far remain unanswered: will we continue to live with the SARS-CoV-2 virus, which causes the covid-19 infection? When will we achieve herd immunity? How long does immunity last? To understand how the covid-19 pandemic will evolve and how best to deal with it, knowledge of existing vaccines and the experience of past infections / pandemics must be considered.
1) SARS-CoV-2 will stay with us, but it will become endemic - like a coronavirus with recurring seasonal peaks. We have long known four coronaviruses that activate in winter and infect people, causing mild colds. In addition, since 2003, we have been sick with two completely new coronaviruses that can cause extremely serious infections: SARS-CoV-1 (causes severe acute respiratory syndrome with a mortality rate of about 10%) and MERS-CoV (Middle East respiratory syndrome with a mortality rate of about 34 %).
The key question is which way SARS-CoV-2 will go. Will it adapt better to humans through evolutionary selection (through gradual mutations), as the four seasonal coronaviruses have done over the centuries? Or will it fail to adapt like SARS-CoV-1 and MERS-CoV, which cause severe disease but have limited transmission?
Like the four seasonal types of coronavirus, the novel SARS-CoV-2 coronavirus is showing global spread and its lethality is likely to decline over time. For a virus of animal origin to cross the species boundary and pass to humans, mutations are required, in which the adaptation of the virus often occurs due to a decrease in pathogenicity (compare the more infectious strain "Delta" with "Alpha", which, in turn, spreads more efficiently than the original Wuhan).
A similar example is the weakening of the pathogenic ability of viruses by cultivation in the laboratory in the production of live attenuated (attenuated) vaccines. The same adaptation of viruses can take place in nature: viruses do not benefit from causing severe illness to the detriment of the effective spread of infection, but they also benefit from certain symptoms - for example, increased production of aerosols, diarrhea or vomiting.
Thus, everything suggests that SARS-CoV-2 is becoming endemic, and in the future we will have to learn to live with it.
2) It will not be possible to eradicate SARS-CoV-2. At the beginning of the pandemic, there was an opinion that SARS-CoV-2 could be eradicated using natural immunity or the so-called “herd” or herd immunity through vaccination. However, history shows that only viruses can be completely destroyed, whose only host can only be a person. The best example of this is smallpox, which was completely eradicated in the 1970s thanks to vaccination.
Thus, zoonotic viruses (transferred from animals) like SARS-CoV-2 are unlikely to be destroyed due to their origin and natural characteristics.
3) SARS-CoV-2 will generate new strains with increased infectivity for a long time. It is likely that the original SARS-CoV-2 strain originated from bats. At some point, the virus spread to humans and we got the first outbreak in Wuhan at the end of 2019.Presumably, this was possible because the bat virus mutated in a so-called spike protein, which facilitated binding to a human target cell, ACE-2 (angiotensin converting enzyme 2 or ACE-2, the main binding molecule for SARS-CoV-2).
Alas, the virus will continue to mutate in the future. During the pandemic, we constantly observed various mutations and the emergence of new strains. Some of them are already highly infectious, but most of them are not transmitted further due to the fact that new mutations are not useful enough for the virus itself.
4) It will take a long time for SARS-CoV-2 to reduce its pathogenicity. Seasonal coronavirus has been spreading for a long time. How and why these zoonotic viruses were transmitted to humans and took root is largely an open question.
An interesting example, somewhat similar to SARS-CoV-2, is the seasonal OC43 coronavirus, which appears to have been transmitted to humans from livestock in the 1890s. OC43 was first discovered in 1967 in children with cold symptoms.
Thus, history suggests that it will take many years before SARS-CoV-2 turns into a seasonal coronavirus, mainly affecting children.
5) Modern vaccines will provide protection against severe and minor diseases caused by currently known strains of SARS-CoV-2. In total, there are seven different coronaviruses that cause disease in humans. Like SARS-CoV-2, rubella virus, mumps virus (mumps) and measles virus are also monotypic, however, despite prolonged mutations, we still have very effective vaccines for all three viral diseases.
Interestingly, the vaccine strains for all three have not changed since the 60s and 70s. Despite mutations, new types of viruses do not take root.
6) It will hardly be possible to reach herd immunity. Many virologists are now of the opinion that we are unlikely to achieve herd immunity. The argument is based on the fact that herd immunity has historically been achieved not through natural infection, but through vaccination.
It is important to note that whether it is caused by infection or vaccination, immunity to respiratory viruses (including coronaviruses) is extremely short-lived.
Collective immunity arises when a large part of the population becomes immune to the disease - that is, the transmission of the disease from person to person is limited or stops altogether due to the absence of susceptible people in the environment of the infected patient. As a result, the whole society is protected - not only the vaccinated, but also the unvaccinated.
At the beginning of the pandemic, many believed that 50-70% antibody levels in the population would be enough to stop infection, but several recent outbreaks have shown that even a very high vaccination rate (over 90%) does not guarantee that transmission will stop.
We are convinced that it will be very difficult or even impossible to achieve herd immunity because:
- Modern vaccines for SARS-CoV-2 provide protection against death and serious illness, but not against the spread of the infection itself.
- Studies of seasonal strains of 1970s coronaviruses have shown that immunity lasts less than a year on average, so relapses are extremely common. Similar observations are beginning to appear in relation to people vaccinated against SARS-CoV-2.
- It is unlikely that a vaccine will provide longer lasting protection than natural infection - despite the lack of information on new mRNA vaccines.
- Globally, only a small part of humanity has been vaccinated so far, which means that for a long time there will be many unvaccinated in the population.
It is important to note that immunity from respiratory viruses, including coronavirus, whether due to previous infection or vaccination, is extremely short-lived.
Lennart Svensson, Professor of Molecular Virology at Linkoping University, Former Chairman of the Swedish Society of Virology
Oke Lundqvist, Professor of Virology, Uppsala University
Anders Wiedell, Associate Professor of Clinical Virology, Lund University