Novavax Inc. shares NVAX, -0.90% slid 69% in premarket trade Thursday, after the company said a trial of a treatment for the main cause of viral pneumonia in infants and young children failed to meet its main goal. Gaithersburg, Md.-based Novavax said the phase 3 trial of ResVax, an aluminum adjuvanted respiratory syncytial virus (RSV) fusion (F) protein recombinant nanoparticle vaccine, did not meet its primary endpoint of preventing medically significant RSV lower respiratory tract infection. The trial did show efficacy against a secondary objective of preventing hospitalization, however and is the first RSV vaccine to show phase 3 efficacy. The vaccine is being developed to protect infants by immunizing mothers against the disease, the leading viral cause of severe lower respiratory tract disease in infants and young children. RSV is the leading cause of hospitalization of infants in the U.S., with an estimated annual rate of up to 76,000, the company said. "Importantly, while this study did not meet the pre-specified success criterion for the primary clinical endpoint of this trial, the data indicate that ResVax protects infants from some of the most serious consequences of RSV, including RSV LRTI hospitalizations and RSV LRTI with severe hypoxemia," Novavax Chief Executive Stanley C. Erck said in a statement. The company is planning to meet with key regulatory authorities to discuss its next steps. Novavax shares have fallen 1.8% in the last 12 months through Wednesday, while the S&P 500 SPX, +0.14% has gained 2.9%.