Will a Vaccine Work?

Last year’s seasonal flu vaccine was judged 45% effective. This is typical effectiveness for a seasonal flu vaccine after half a century of practice. Now let’s say 50% of the people will take the vaccine in line with recent studies. This is multiplicative so the population effectiveness is 22.5%. SARS 2 has an Ro of at least 3, so a vaccine would need to be effective for 70% of the population. 22.5% will help but will not end the pandemic. However, natural immunity from recovery has been building. Unlike vaccine immunity based only on who was given or will take the vaccine, natural immunity is based on the virus finding the most susceptible first. Natural group immunity may require only ~20% of the population to end the pandemic. The extent of natural immunity is currently unknown. The pandemic will end when either the natural or vaccine threshold of group immunity is achieved, or some combination thereof. What is clear is that a significant contribution from natural immunity will be necessary for a vaccine to work.

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