Shares of Pfizer Inc. /zigman2/quotes/202877789/composite PFE -2.33% gained 0.1% in premarket trading on Wednesday after the drug maker said the "totality" of clinical data indicates that people who are at least 16 years old and were vaccinated with the COVID-19 shot that it developed with BioNTech SE /zigman2/quotes/214419716/composite BNTX -5.77% should get a booster after six months. In a document published Wednesday morning by the Food and Drug Administration in advance of an advisory committee meeting set for Friday, Pfizer said its Phase 3 substudy examined 306 people between the ages of 18 and 55 years old who got a third dose. It found that an extra dose is considered safe and increased neutralizing antibody titers against the original strain of the virus as well as the beta and delta variants to a higher rate than what was reported after two doses in the clinical trials last year. Younger participants were more likely to report systemic events after the second and third doses than the first dose; fever was more common after the third dose. No adverse events were reported in the first month after the third dose. Pfizer also cited research conducted by Israel, the Centers for Disease Control and Prevention, and the hospital system Kaiser Permanente in the document, saying that data from Israel "suggest reduced effectiveness against severe disease could eventually follow observed reductions in effectiveness against SARS-CoV-2 infections." Pfizer's stock is up 21.4% this year, while the S&P 500 /zigman2/quotes/210599714/realtime SPX -1.89% has gained 18.9%.