Shares of Pfizer Inc. gained 0.1% in premarket trading on Monday after the company said it is testing its experimental 20-valent pneumococcal conjugate vaccine in combination with a booster dose of its COVID-19 vaccine in adults who are 65 years old or older. The study includes 600 participants from the Phase 3 clinical trial for the COVID-19 vaccine. Both Pfizer and Moderna Inc. are testing booster doses for their two-dose, mRNA-based COVID-19 vaccines. Pfizer’s stock is up 8.5% so far this year, while the broader S&P 500 is up 10.6%.

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