Why don't we have a vaccine for SARS? MERS? HIV? These viruses have been around longer than SARS-CoV-2. A mini-thread to provide some perspective and even a little dose of optimism. We are not in the same situation here. 1/7

HIV is a unique beast, and unfortunately we may never have an HIV vaccine. HIV attacks the host immune system. It also mutates rapidly. Viral diversity within a single person is comparable to diversity of influenza across the globe. SARS-CoV-2 is more stable than influenza. 2/7

SARS caused an explosive outbreak in 2003, but it was contained and hasn't circulated since. While vaccine candidates exist, there has been insufficient funding to test them, and also no pathway for licensure. How can you tell if they prevent SARS if there is no SARS? 3/7

MERS regularly spills over from its camel reservoir. While early outbreaks were explosive, Saudi Arabia has made great improvements to prevent within hospital transmission. As a result, outbreaks are much smaller. They are also very hard to predict... 4/7

At a 2018 WHO meeting, we discussed potential design strategies for a MERS vaccine efficacy trial. But given the relatively low incidence even in high risk groups (camel workers and their families), trials would need 100,000+ participants. 5/7
https://www.who.int/blueprint/what/norms-standards/meeting-report-30-november-2018.pdf?ua=1

Organizations like @CEPIvaccines have been funding research for MERS vaccine candidates, but for now there still isn't a clear path forward for getting it approved by regulators. 6/7 https://cepi.net/research_dev/priority-diseases/

So certainly it would be *amazing* to have effective SARS, MERS, and HIV vaccines, but I do not view the lack of approved vaccines as indicating that we are unlikely to find one for SARS-CoV-2. Particularly because we are pursuing so many different candidates at once. 7/7 END