Enlarge /. Face masks could become an integral part in the near future.
Most optimistic ideas about what to do with SARS-CoV-2 relate to the extinction of the virus. We could speed up the tests and isolate anyone who has come into contact with an infected person. We could carefully treat infections to build herd immunity without exceeding our hospital capacity. In an ideal world, we could develop herd immunity with an effective vaccine.
Unfortunately, there are reasons to worry that none of them will work. Tracking the contacts of infected people may not be possible with a virus that spreads as easily as SARS-CoV-2. And some of the virus' s closest relatives do not build up the long-lasting immune response required for persistent herd immunity. All of this raises a worrying question: what happens then?
A group of Harvard epidemiologists attempted to answer the question by trying models that tested the effects of various assumptions about the behavior of the virus and the response of the immune system to it. The researchers note that there is a risk that it will become a seasonal threat and that we may need to isolate ourselves socially every winter.
The study is based on what we know about the closest evolutionary relatives of SARS-CoV-2. Corona viruses are a family-level label, two steps away from the species. The genus is a step of the species and there are four types of coronavirus (alpha, beta, gamma and delta). SARS-CoV-2 is a member of the beta coronavirus, a genus that includes people with previous pandemic fears such as SARS-CoV-1 and MERS. It also includes two types that are less threatening and more annoying: HCoV-OC43 and HCoV-HKU1, which together are the second most common cause of cold-like symptoms.
The reason why these cold viruses cause so much trouble is that they don't create long-term immunity. A year after infection, people's immune systems seem to have forgotten that they have ever seen the virus.
However, there are complicated relationships between the responses generated by these beta corona viruses. SARS-CoV-1 produces a long-lasting immune response that may contain antibodies that block the cold viruses HCoV-OC43 and HCoV-HKU1. While the immune response to the cold viruses is weak, the antibodies raised against them react to SARS-CoV-1.
To find out what these interactions might mean, the researchers took test results for these two viruses and created an epidemiological model that matched their prevalence. As expected, the model showed a seasonal infection pattern with maximum rates between October and May. The model also suggests that the arrival of autumn causes the peak to begin, but its decline is largely due to the lack of vulnerable people, as much of the population was infected in the spring. In addition, the results indicate that infection with one of the viruses offers some protection against the second, which means that only one is common in most years.
Now add a pandemic
We also have SARS-CoV-2 this year. The basic properties of the virus – how long it takes to become infected, how long it is infectious, etc. – was based on the properties that we see in different countries. However, the most important questions here cannot be extracted from known data: how long does the immune response last and whether it offers protection against related coronaviruses. Another open question is whether the virus may have seasonal behavior, as we see it from its relatives.
So the researchers simply tried different values for these properties to see what would happen.
In all cases where immunity to SARS-CoV-2 is not permanent, the virus can cause sporadic outbreaks. If the duration of immunity is less than a year, the outbreak occurs annually. If it takes longer, COVID-19 cases could break out every two years. The model shows that we have to develop long-term immunity in order to actually have the chance to suppress future outbreaks.
Cross immunity has some interesting implications. If the common cold viruses have poor immunity to SARS-CoV-2 (around 30 percent), it is enough to delay future outbreaks of COVID-19. For example, if SARS-CoV-2 had its next outbreak in 2022, this cross immunity would postpone it to 2023. If SARS-CoV-2 induces immunity to the common cold virus, the effects could be dramatic. A cross immunity of 70 percent would be sufficient to effectively eliminate the circulation of the cold virus.
Of course, we don't just let SARS-CoV-2 circulate freely. When the researchers added social distancing efforts, they saw what had been seen in other models: infections were suppressed, but the virus returned with all its might after it was picked up. And because isolation so effectively suppresses the virus's circulation, little immunity has been built up in the population. As a result, the following outbreak is about the size of one that never tries to distance itself socially. Adding a seasonal impact on the behavior of the virus would simply ensure that the outbreak would occur in winter after the distance.
When assuming that distance rules were put in place as soon as the infections reached a certain level, the author's model suggested that the current outbreak could last until 2022, with social distance rules applying in 25 to 75 percent of cases. During these years, the gaps between the times when distance is enforced would widen as the percentage of the population with some immunity would increase steadily.
The researchers also looked at two things that could reduce the social impact of the ongoing outbreak: increasing the capacity of intensive care and partially effective therapy. Both of these could have a major impact, as it would allow us to avoid social distance for longer without overloading the health care system. And that would mean that we would tolerate more infected people, and the resulting immunity would also be an advantage.
While there is a lot of information here, there are a few key takeaways. The key ideas to combat SARS-CoV-2 are to create herd immunity, either through controlled infections or through a vaccine. However, these require permanent immunity that is far from guaranteed. This doesn't mean that a vaccine won't work, but it does mean that we may need to plan annual boosters – maybe it could be converted into a flu vaccine. Even without a vaccine, the model suggests that SARS-CoV-2 could engage in behavior that resembles our annual flu outbreaks.
The other big advantage is that we need to know how long the immunity lasts and if there is cross-immunity with the other strains of beta coronavirus to really understand what's next. Differences in these characteristics lead to very different behavior at the population level. And that means finding the actual values of these properties should be a priority if we want to plan intelligently.
Of course, as with any other model, there are limits. As with all other models currently in operation, it depends on our incomplete knowledge of things like the infectivity and frequency of asymptomatic cases. This particular model is also limited by its treatment of the population, which does not contain details of the geographic distribution of this population or its interaction modes.
And more generally, it is important to emphasize that no single model will ever be an exact recap of reality. Instead, these models only show the rough outline of what we should expect if a certain list of assumptions turns out to be correct. And ideally, we get a stronger consensus on what our short-term future will look like when more models deal with overlapping issues.
Science, 2020. DOI: 10.1126 / science.abb5793 (About DOIs).