New modelling led by interdisciplinary scientists at the University of British Columbia (UBC) could help public health officials better factor in the long-term impacts of infectious disease when determining optimum vaccine coverage rates.
"Our models indicate even small post-infection costs can tip the scales and make the case for vaccine-driven elimination," said Dr. Chadi Saad-Roy, an assistant professor in UBC's Department of Mathematics and Department of Microbiology and Immunology and senior author of the new study, published this week in the Proceedings of the National Academy of Sciences.
The COVID-19 pandemic highlighted how the effects of an infection can linger, with more than 400 million people globally experiencing persistent symptoms ("long COVID"). While evidence of post-viral impacts is mounting, models that evaluate infectious disease management strategies typically focus only on the acute phase.
To address this gap in the current study, the researchers developed a system of mathematical equations to examine interactions between interventions and post-infection effects.
"From the perspective of society, there's some cost to producing vaccines, and there's a big benefit to reducing infections and post-infection effects. So, the question is: Where does this optimum happen?" said Dr. Saad-Roy. For a benign virus, it may not be worth it to vaccinate at all. For other infections, it may best to vaccinate enough people to eliminate the disease. Or the ideal point may fall somewhere in between. "Our modelling can help answer those questions."
The team first used an epidemiological model that calculates the fraction of susceptible, infectious and recovered individuals to evaluate non-pharmaceutical interventions that reduce transmission (such as masking, social distancing and air filtration) as a complement to vaccines.
They found that, on long timescales, non-pharmaceutical interventions substantially decreased the portion of people who became infected and then recovered—and were therefore vulnerable to post-infection effects), suggesting that these measures can effectively reduce long-term disease burden.
Next, the researchers coupled epidemiological and economic models to assess how the cost associated with post-recovery syndromes affects the optimal level of vaccine coverage.
To provide a tool that can pinpoint this sweet spot, the researchers wrote equations that account for the societal cost of acute and long-term infection (e.g. cost of hospitalization and loss of labor force) and the cost of vaccination in terms of development, production, distribution and recruitment.
They looked at three different vaccine cost scenarios as the number of vaccines given increases: one in which cost accelerates (assumes that there is a first wave of easy-to-reach vaccine adopters and that it becomes harder to vaccinate additional people due to vaccine hesitancy or logistical barriers), one in which cost decelerates (assumes that while there is initial resistance to vaccination, influence from peers makes it easier to vaccinate additional individuals) and one in which cost alternately accelerates and decelerates.
The team found that, in all scenarios, the ideal amount of vaccine coverage was very sensitive to post-acute effects and that even small post-infection costs could shift the optimal coverage to higher rates and even to elimination levels.
"We need to tackle this rigorously and not dismiss individuals' experiences as anecdotal," said Dr. Saad-Roy. To more systematically monitor and document post-infection effects, researchers may conduct large, long-term cohort studies that survey participants on a broad constellation of symptoms. "These strategies can effectively reduce acute infection, which in turn mitigates post-acute burden and ultimately leads to a healthier world."
It's critical to understand which vaccine cost scenario—accelerating, decelerating—is at play in a specific setting in order to determine the optimal coverage level. This will require socioeconomic studies that explore vaccine decision-making dynamics in a given time and place, which may be influenced by outreach efforts.
"If we can educate people about the benefits of vaccination and reduce hesitancy, that can lower the cost of vaccination and tip the balance in favour of vaccinating to eliminate the disease," said Prakhar Jaiswal, a graduate student in the UBC Department of Mathematics and first author of the study.
The interdisciplinary team includes co-authors Daniel Coombs (UBC Mathematics), Caroline Wagner (McGill Bioengineering) and Troy Day (Queen's Mathematics & Statistics). This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canadian Institutes of Health Research (CIHR).
With files from Sarah Anderson, UBC Microbiology and Immunology.
