Question: One of my friends told me that while studies have shown that the Covid vaccines are effective in preventing a vaccinated individual from getting sick with Covid, it is currently unknown whether or not a vaccinated individual can still get and shed the virus – potentially transmitting the virus to other, unvaccinated individuals. Is this accurate?

Answer: Your friend is correct.  The Pfizer, Moderna, and AstraZeneca trials are primarily aiming to understand whether the vaccine protects against illness (COVID-19).  For example, the Pfizer trial’s main endpoint is cases confirmed ≥7 days after Dose 2– with a case defined as a positive virologic test plus at least one COVID-19 symptom (e.g. symptomatic cases).  The initial data we’re seeing from all three vaccine trials is a resounding yes– these vaccines can prevent illness!  That’s so exciting, but many questions remain*, including whether the vaccines can prevent infection:

1. Does the vaccine prevent infection? Secondary efficacy endpoints vary by study, with Moderna and AstraZeneca also examining whether the vaccine prevents infection.  The Pfizer trial does not include this secondary endpoint.  As FDA wrote in its review of the Pfizer vaccine, “Data are limited to assess the effect of the vaccine against asymptomatic infection… Additional evaluations will be needed to assess the effect of the vaccine in preventing asymptomatic infection, including data from clinical trials and from the vaccine’s use post-authorization.”
2. Does the vaccine prevent transmission? More data is required. As FDA writes in its review of the Pfizer vaccine, “Data are limited to assess the effect of the vaccine against transmission of SARS-CoV-2 from individuals who are infected despite vaccination. Demonstrated high efficacy against symptomatic COVID-19 may translate to overall prevention of transmission in populations with high enough vaccine uptake, though it is possible that if efficacy against asymptomatic infection were lower than efficacy against symptomatic infection, asymptomatic cases in combination with reduced mask-wearing and social distancing could result in significant continued transmission.”
3. How long does immunity last? So far, we have solid data from the Pfizer vaccine that immunity lasts at least two months (length of the current analytic period).  For the current randomized control trials, Pfizer and Moderna plan to follow participants for 2 years after the second dose while AstraZeneca plans to follow participants for 2 year after the second dose. 
4. How well does the vaccine work among some high-risk populations? While high risk populations are included in the vaccine trials, some groups may be represented in too small numbers to draw conclusions (see Q&A of 12/10) 
5. How well does the vaccine work among children? For the Pfizer vaccine, we cannot extrapolate to children <16 years of age.  Expansion of trial to younger ages and ongoing data collection among children ages 12+ will fill these knowledge gaps. (see Q&A of 12/9)

*For the longer list of outstanding questions, see pages 46-28 of FDA’s review of the Pfizer vaccine.