India, Aug. 20 -- Moderna shares fell about 12% in pre-market trading on Thursday, one day after the stock jumped 177%. The drop came even after Moderna and Merck announced a major success in their cancer vaccine trial.

Moderna and Merck said their personalized mRNA cancer vaccine, intismeran, met the main goal of a Phase 3 trial. The vaccine was tested in melanoma patients who had already had surgery to remove their tumors.

The trial found that adding intismeran to Merck's cancer drug Keytruda helped patients stay longer without their cancer coming back or spreading. But after the huge jump in Moderna's stock, investors are now looking at what this success could mean for the company's business and future earnings.

The Phase 3 trial co...