As public health agencies strategize about how to distribute COVID-19 vaccines after they are developed, I believe that research on cholera vaccination strategies may be able to provide insight into how to make the best use of limited vaccine supplies. Many known factors about cholera (for example, duration of immunity and seasonality) are still unknown for COVID-19. Furthermore, the progression of both diseases, their modes of transmission, and their dynamics are different. Nevertheless, I believe that principles from cholera vaccination campaigns can be applied to COVID-19 control for two key reasons. First, many strategies for controlling cholera account for limited vaccine supplies and these approaches may be applicable immediately after COVID-19 vaccines are developed. And second, because both diseases can spread rapidly throughout the population, the timing of vaccination will be essential.
Cholera outbreaks can quickly lead to a public health crisis through their rapid spread in vulnerable populations. Although adequate access to water, sanitation, and hygiene (WaSH) can most sustainably prevent cholera transmission, vaccination provides temporary immunity and is especially useful in areas that lack WaSH infrastructure. A global stockpile of the oral cholera vaccine is available and can be used to help contain or prevent outbreaks. Although there have been recent efforts to increase supply, the stockpile has historically been limited. This has forced the World Health Organization to distribute less vaccine than requested. Requests are granted based on various factors such as the risk of cholera, severity of an outbreak, and access to WaSH and healthcare. These factors can be used to assess the vulnerability of a population to a severe, widespread outbreak. Importantly, there is a delay from the time a country requests the vaccine to when public health practitioners actually administer it, and this can have large effects on the trajectory of an ongoing outbreak.
A good candidate for vaccination might be a population in which the level of immunity is low and the risk of transmission is high.
The limited resources of the global stockpile have prompted modeling studies to consider optimal ways to distribute cholera vaccines to populations. Back in 2012, I worked with the US Centers for Disease Control and Prevention, the Thai Ministry of Public Health, and Première Urgence Aide Medicale Internationale to prepare for an oral cholera vaccination campaign in Maela, the largest Burmese refugee camp in Thailand. As part of this, we developed a transmission model to examine the effects of the campaign and also to examine theoretical vaccination strategies if doses had been limited. Consistent with previous findings for cholera and other diseases, we found that the most effective method to avert cases for a given amount of vaccine involves vaccinating a population before an outbreak would occur.
Generally speaking, this can be applied to other settings, especially in regions in which cholera outbreaks are periodic (that is, each rainy season) and when access to WaSH infrastructure is limited. However, it is not always easy to predict a future outbreak. In line with previous findings, our research also revealed that when an outbreak is ongoing, the timing of vaccination is critical because of the rapid spread of cholera. In fact, we found that although the full two-dose vaccine regimen confers more immunity than a single dose, it may be preferable to give one dose to more people rather than two doses to fewer people, because two doses must be administered 14 days apart and would incur a longer delay. In general, if a vaccine is administered too late in an outbreak, the number of cases that it prevents will be substantially reduced.
Immediately after the development of a COVID-19 vaccine, supplies will likely be limited. During this time, groups that are at high risk for becoming infected or having complications from COVID-19 (for example, healthcare workers and elderly individuals) will likely be vaccinated. In addition to these at-risk groups, populations that are most vulnerable to a widespread outbreak should be prioritized for vaccination. Specifically, a good candidate for vaccination might be a population in which the level of immunity is low and the risk of transmission is high, for instance, due to high population density. This is analogous to the early stages of a cholera outbreak, when there is a large proportion of people susceptible to infection. Estimates for the level of COVID-19 immunity in a population can be informed by seroprevalence studies, however, more understanding about the correlation between antibodies and immunity is needed.
There are many other considerations for identifying the vulnerability of populations to a widespread outbreak. For instance, if access to healthcare and treatment is limited, or people are unable to socially distance, vaccination could be used to decrease risk in the population and potentially prevent future outbreaks. This is similar to administering cholera vaccine to populations that have limited access to WaSH infrastructure. It is also essential to account for the logistical challenges and delays when deploying the vaccine to populations with ongoing outbreaks. As with cholera, any delays in the early stages of an outbreak may substantially increase the total number of cases and deaths.
Joshua Havumaki is a postdoc at the Yale School of Public Health in the Department of Epidemiology of Microbial Diseases.