Love Thy Neighbor(s): The Need for Herd Immunity

As we posted recently, India had its first polio-free year, despite significantly lagging behind in other vaccinations. The economic losses of vaccine-preventable diseases (VPD) to developing countries are tremendous: investing in vaccines in low- and middle-income countries would save 6.4 million children until 2020 – an investment valued at $231 billion. The same is true for developed economies: the United States has saved over $180 billion by becoming polio-free.The value of disease eradication is very high because VPD elimination is a global public good: once a VPD is eliminated, everyone benefits from it. This also brings in the incentives to free ride, given the nonrivalrous and nonexclusive nature of global public goods. The economic and humanitarian benefits of vaccination are evident; but so are the economic limits of achieving elimination.Thus, the discussion shifts from elimination to control, or achieving herd immunity: the threshold for vaccination rates above which disease cases become highly infrequent. Herd immunity levels for several diseases are achieved when vaccination rate goes up to 85%; yet vaccination rates in most low- and middle-income countries are significantly below that, according to household survey data. Herd immunity against most vaccine-preventable diseases benefits all countries by reducing the frequency of imported outbreaks.What would be the best way to go to achieve herd immunity, given the resource constraints? A study by Klepac et al, recently featured in The Economist, finds that the optimal vaccination rate depends on the relative costs of vaccination, and not on the level of contagiousness. Coupled with porous borders and immigration, the burden of each additional infection exceeds the cost, pointing out to the need for sustained investment. This inter-connectedness also promotes free-riding in vaccination efforts between populations and results in lower levels of vaccination in each subpopulation relative to the global optimum. Given this, it is important to pool resources together, especially for neighboring communities, to ensure herd immunity: India, for example, would benefit from supporting vaccination efforts in Bangladesh as much as it would benefit from increased uptake within its own borders. (Granted, India performs worse than Bangladesh on vaccination, so India probably should be thanking Bangladesh for doing the right thing.) The same is true for many countries in South and Southeast Asia, where neighboring countries have low vaccination rates.This is why vaccination should be perceived as a global public good across the world: achieving herd immunity requires coordinated action to increase vaccination levels.