When it comes to predicting the future development of the Covid 19 pandemic, many factors are still unclear: How long does the immune protection last after an illness has been overcome? Could a second infection with the virus be milder? How well will a vaccine protect? Which additional measures – such as a mask requirement – remain necessary? Researchers have now integrated all of these factors into an epidemiological model. Depending on how they vary the individual parameters, there are very different courses: from further severe waves of infection to almost complete containment of the pandemic.
The Sars-CoV-2 coronavirus has been spreading worldwide since the beginning of 2020. To curb the transmission of the virus, many countries have put in place political measures, including curfews, shop closings and masking requirements. In addition to these non-pharmaceutical interventions (NPIs), research into a vaccine is in full swing. So far, however, it is difficult to predict how the pandemic will develop. The extent to which a survived infection or vaccination generates immunity against the virus will play a decisive role. In the best-case scenario, a single infection or vaccination would leave lifelong immunity; in the worst-case scenario, antibodies from previous Sars-CoV-2 infections would even worsen the course of a new infection. However, previous studies indicate that a coronavirus infection that has been overcome offers a certain protection against future infections, but this may not be complete.
Models for immunity
Researchers led by Chadi Saad-Roy from Princeton University have now reproduced the various immunity scenarios using epidemiological models and simulated them from the start of the pandemic to around five years in the future. On the one hand, they used the so-called SIR model (Susceptible-Infected-Recovered), which assumes complete immunity, and on the other hand, the so-called SIRS model (Susceptible-Infected-Recovered-Susceptible), in which people recovered after a certain time become susceptible to the virus. They incorporated numerous other factors into these models, including seasonal fluctuations, non-pharmaceutical interventions, and the availability of a vaccine. They also examined how people who refuse masks, vaccinations and similar measures affect the course of the pandemic.
As expected, the models show that the initial pandemic peak is largely immune to immunity, as most people are vulnerable. Here NPIs such as protective masks and social distancing play a particularly important role. If the Sars-CoV-2 infections increase – and with it the immunity in the population – the models differ considerably, however, depending on the protective effect of the diseases that have been overcome: “If the immune responses are weak or only temporarily protect against renewed infection , then larger and more frequent outbreaks can be expected in the medium term, ”says Saad-Roy’s colleague Andrea Graham. Saad-Roy adds: “In the short term and during the pandemic phase, NPIs are the key determinant of how many cases there are. The further into the future, however, the more important the role of immunity becomes. “
Weak vaccines can help too
As the models show, vaccines can have a significant impact on the future dynamics of the pandemic. As expected, it would be particularly positive if the vaccine offered long-lasting and as complete protection as possible. But as the data show, weaker vaccines can also help reduce the number of infections if a sufficient number of people are vaccinated. The researchers estimate that such a vaccination will be available around 1.5 years after the start of the pandemic. It is true that individual vaccinated persons could still fall ill in the further course, but since more and more people are at least partially immune, the spread of the virus would be slowed down.
However, if many people refuse to be vaccinated, this could hamper containment of the pandemic. This is especially true if the same people also reject non-pharmaceutical measures and, for example, gather together unprotected. “If vaccination rejection is correlated with increased risk behavior, such as refusal to wear a mask, the vaccination rate necessary to achieve herd immunity is much higher,” says co-author Simon Levin of Princeton University.
Online simulation available
The models presented can only depict the infection process in a simplified manner and do not include all conceivable factors. “With so much uncertainty in the underlying processes, it is difficult to make accurate predictions about the future,” says Senior Author Bryan Grenfell. “We argue in this study that ultimately a combination of simple and more complex models is the best way to deal with these circumstances. Carefully comparing the forecasts of different models and deriving an averaged picture of the future from them – as with the weather forecast – can be very helpful. “
According to the researchers, it is not only important to record how many people are sick, recovered or susceptible, in order to follow the infection process further and thus create a more solid database for prognoses. In addition, examinations of antibody levels in recovered persons are required as well as further research on immunity after surviving Covid-19 disease and on possible cross-immunities with colds.
The researchers provide their models on the Internet to disposal. The interactive online tool makes it possible to vary factors such as immunity, protective measures and vaccination effectiveness and to display the course of the pandemic under the set conditions.
Source: Chadi M. Saad-Roy (Princeton University) et al., Science, doi: 10.1126 / science.abd7343