While it is undoubtedly a substantial undercount, as of April 20th the CDC has been notified of 7,157 breakthrough cases – a roughly 25% increase over their initial report of April 13th.
This is a reassuringly low number when compared to the nearly100 million fully vaccinated individuals in the country, but it is a reminder that vaccination is not an ironclad guarantee of protection. Hence the need for vaccinated individuals to continue to observe some of the same social distancing measures as those who remain unvaccinated.
These breakthrough numbers will undoubtedly rise substantially over time. Even if our current vaccines should prove to be 90% effective in preventing COVID infection – and no one is claiming that – millions of breakthrough infectious would still be expected over the next year or two.
Where COVID vaccines are expected to have their greatest impact is in reducing severe disease, which should manifest itself in lower hospitalizations, and far fewer deaths. How effective they will be in preventing mild or asymptomatic infection remains to be seen.