Written by Dr Flavio Toxvaerd

Waning immunity and the second wave: projections for COVID-19

In a recent paper projections of future transmission dynamics for COVID-19 are offered in a SEIRS model in which immunity is waning over time. The model simulates how the optimal policy (with the three distinct features: mitigation, delay and non-eradication) depends on how fast immunity wanes and on the infection fatality rate. Will partial lockdown and social distancing become the future norm?

As we reach three months since COVID-19 was declared a global pandemic, public discourse around the world centres around getting through the worst part, with little attention paid to what happens in the medium and longer run. But as of now, we cannot be sure that immunity to SARS-CoV-2 is permanent, and so society and governments must look ahead and prepare for this possibility.  If it turns out that immunity is waning, it is not enough to only deal with the current situation and postpone dealing with the new situation until it arises; what we do today influences what happens next and thus the future impact of current decisions must be factored into policy design now.

In a recent working paper with epidemiologist Stephen Kissler, we explicitly consider and analyse a setting in which immunity is only temporary. We consider a type of model known as an SEIRS model, which stands for susceptible-exposed-infected-recovered-susceptible. This is a flexible modelling framework well-known in the field of epidemiology, but which has only recently received attention within the economic analyses of epidemics. In such a framework, we find that because immunity may slowly disappear from recovered people, there is the potential for a second (and even third) wave of infection. Based on what we know from medical science about other related coronavira, the second wave could lag the first wave by anything from six months to two years. Factoring in this possibility has very important consequences for how health authorities should manage the disease.

In our work we couch the analysis in terms of conditions prevalent in the United states, using the relevant numbers for births, deaths and population size, in addition to the most up-to-date estimates of transmission, infection and death rates relating to COVID-19. We then consider how a health authority would go about controlling the epidemic through social distancing measures, such as a partial lockdown of the economy. We find that when immunity is only temporary, the disease will become endemic and the optimal policy, after an initial but significant effort to reduce the first great infection wave, is to engage in a permanent low level management of the persistent infection in the population, in order to keep it under control. Notably, this may mean that the disease is never completely eradicated.

In practice, this means that partial lockdown or social distancing measures may become the norm. What may such measures entail? There are many practical measures that can be considered. In public transport, additional capacity of trains and buses could ensure that travellers can commute to work while observing suitable social distancing. Schools may operate rotation schemes with morning and afternoon classes for senior years or rotation for students to attend only one some days. Measures such as the wearing of facemasks in public spaces could become ubiquitous, as could shifts in social norms such as greeting by the shaking of hands. There will also be a need to change how businesses operate, with a need to change layouts in retail spaces, shared office spaces, factory floors etc.

An underlying assumption in our analysis is that in the short run, the only policies at our disposal are broad-based non-medical interventions such as social distancing and lockdown measures. At the initial stages of the COVID-19 pandemic such policies proved to be extremely costly from a social, economic, medical and other perspectives. But going forward, we expect that individuals, businesses and governments are likely to adapt how they do things and operate to mitigate the costs of this initial dramatic shock. People may become more cautious in everyday dealings, businesses may come to depend less on third parties or off-shoring, while other organisations such as schools, transport, intermediate goods producers and local government may find innovative ways to become more flexible and resilient in the ways in which they deliver their services and products. And so, to end on a positive note, we hope that with creativity and resourcefulness, humanity will learn to navigate and live with the disease, should it turn out that it is here for the long term.


Click HERE for the Working Paper, 'Waning Immunity and the second wave:some projections for SARS-CoV-2'

  • About the author

    Dr Flavio Toxvaerd, Visiting Fellow 2018-19

    Dr Flavio Toxvaerd is a University Lecturer at the Faculty of Economics, University of Cambridge and a fellow of Clare College. He holds degrees from the University of Copenhagen (BSc Economics, MSc Economics), the London School of Economics (MSc Econometrics and Mathematical Economics) and the ...   Learn more

    Flavio Toxvaerd