What Evidence Do We Need to Move Forward With COVID Boosters?
On Monday, employees of Pfizer met with high level executives in the Biden administration to discuss the role of boosters -- a.k.a. a third vaccination with an mRNA vaccine for SARS-CoV-2. Some have speculated that, as with the first two doses, the emergency use authorization pathway will again be used to market boosters. With the rise of the Delta variant and others, enthusiasm in the media and the Twitter commentariat for boosters is growing. However, there are certain criteria that must be met before we jump on the booster bandwagon. Some of these criteria apply at home, and others apply abroad. Abroad  As a general rule, if your goal is to avoid variants -- or mutated versions of a virus -- you want the virus to replicate less. When it comes to variants, it doesn't matter where the virus does the replicating. In a globally connected world, it is only a matter of time before an advantageous mutation finds its way to all parts of the world. As such, we in the U.S., are only as safe as the least safe place in the world. What this means is that before we shift our manufacturing capacity to develop boosters for the current variants, we must make a real effort to ensure that the vaccines we do have get distributed to the greatest number of global citizens who will take them. I argued in April that, practically, this means that children in high income nations should be vaccinated after older citizens globally  this same logic extends to boosters. Before we shift our manufacturing to booster production, we should make sure that we have manufactured adequate supplies of the original vaccine for all global citizens. Moreover, we need to put effort toward solving the last mile problem: how to deliver very cold mRNA vaccines to places in the world where it is difficult to deliver and keep things very cold. This is a technological problem well within our scope. Efforts to manufacture and deliver vaccine boosters to already vaccinated individuals in high-income nations cannot take priority and must not interfere with efforts to vaccinate at-risk individuals around the world. In fact, it is in our best interest to vaccinate those at-risk first. If we pursue boosters in the U.S. without helping the rest of the world, then we might as well get ready for the fourth, fifth, and sixth boosters. We will watch rising death tolls around the world, while worrying that yet new variants may end up on our shores. At Home  Here in the U.S., there are also metrics that need to be met before we contemplate widespread dosing of hundreds of millions of people with booster shots. Specifically: show me the data! I have no doubt that a third mRNA shot will lead to higher neutralizing antibody titers. For that matter, I would guess six shots would outperform three on that metric. But the burden of evidence to accept boosters is not simply a change in antibody titer -- or even demonstration of improved titers for rare variants. We must show that boosters improve clinical endpoints before we ask Americans to roll up their sleeves again. A large randomized trial of vaccinated individuals powered for reduction in symptomatic SARS-CoV-2 or (better yet) severe COVID-19 is needed to justify the harms and inconvenience of boosters. If such a trial simply cannot be powered, or takes a very long time, due to the sparsity of serious infection in the U.S., then the argument for emergency use authorization is inherently flawed. When there's too little disease to run the definitive trial, you are, by definition, no longer in an emergency. One way to solve this problem might be to deliver boosters only in elderly individuals or those who are immunocompromised. Here, a trial measuring COVID-19 outcomes may be possible. Alternatively, a case for boosters can be made if evidence shows that boosters alter the epidemic course for a nation or the globe. Here, too, antibody titers are insufficient. Moreover, ironically, clinical trials would have to be larger and more complex to demonstrate this. For these reasons, I think the burden is on vaccine manufacturers to show that severe COVID-19 outcomes are averted. Finally, we need to consider the second order effects of boosters. A skeptical person may now no longer see SARS-CoV-2 vaccines as the path out of the pandemic, but a recurring, and possibly someday yearly obligation that they may prefer to avoid altogether. We can't ignore the potential impact of discussing boosters on vaccine acceptance. Boosters Without Data  If we accept boosters in the U.S. while the rest of the world remains unvaccinated, and if we authorize them based on inevitable improved laboratory titers without clinical outcomes, we run the risk of creating a medical industrial perpetual motion machine. We will continue to breed new variants outside of our nation, which will lead to calls for yet more boosters, and we will continue to get new boosters without any evidence they are necessary (i.e., lower severe COVID-19 outcomes). Our arms will ache, our hearts will hurt, our wallets will be empty, and so too will our brains, as we will have abandoned all principles of evidence-based medicine. Vinay Prasad, MD, MPH, is a hematologist-oncologist and associate professor of medicine at the University of California San Francisco, and author of Malignant: How Bad Policy and Bad Evidence Harm People With Cancer.