Do the new COVID vaccines prevent transmissions, or only lessen COVID severity (and why should we care)?
The news of vaccine delivery is some of the best of 2020. Pfizer has started distribution and vaccine administration this week in US hospitals. Health care workers will be the first to receive the vaccine and the local and state health departments will be allocating doses to at risk populations as more becomes available.
One question that has come up is does the COVID vaccine prevent transmission? Based on the data, Pfizer estimates the vaccine is 95 per cent effective at stopping people getting sick. But no one knows yet whether the vaccine will keep you from spreading the disease. This NY times article explains this in more detail. This is why people need to keep wearing masks even after vaccination. Until a significant portion of the population is vaccinated (around 70%) we will reach herd immunity as a country.
Vaccine efficacy is a measure of how well a vaccine works in terms of its ability to reduce infection or disease. It’s effectively the percent reduction in disease that you would expect from an unvaccinated population to a vaccinated population.
The type of efficacy that’s been reported for the Pfizer vaccine trials is protective efficacy against disease — in other words, how well the vaccine protects people from getting sick.
Whether or not these vaccines prevent people from getting infected in the first place is a separate question, and not one researcher can answer just yet. Doctors who are part of the study say it’s possible there were people in the trials who received the vaccine, got infected with COVID-19, but didn’t develop any symptoms. Assessing whether a vaccine prevents infection (rather than the development of disease) is particularly challenging to study in humans, partly for ethical reasons.
In these trials, the drug companies waited for people to get sick to determine whether the vaccine offered any protection. But to work out if the vaccine prevents infection, Dr Labzin said researchers would ideally vaccinate people and then deliberately expose them to the virus — to see if they get infected, even if they don’t develop symptoms.
It will take months — if not years — to fully understand how long immunity generated by these vaccines lasts.
The bottom line is, the arrival of multiple vaccines in the next few months is wonderful news but there are many questions yet to be answered.
Prerana Sangani, MD, December 12, 2020