“Even after one dose there seems to be increasing levels of protection, but that protection is further enhanced after the second dose,” said Omer. The data comes from a new CDC study this week that tracked nearly 4,000 US healthcare workers and first responders between doses of the M-RNA vaccines. They were 80% effective after the first dose and 90% after the second. “It appears the vaccinations are also protecting against asymptomatic infection which is also very important,” said Dr. Manisha Juthani of the Yale School of Medicine. She says that’s because it shows the vaccine is working against unknown cases of COVID-19.