Health firm Merck & Co (MRK  ) is ending its COVID-19 vaccine development, citing inadequate immune responses in test subjects. Merck is instead set to focus on the development of drugs to fight COVID.

Merck & Co had been previously working on two vaccines, named V590 and V591, which were developed with the International AIDS Vaccine Initiative's assistance and acquired through the purchase of Themis Bioscience, respectively. According to Merck, however, work on V590 will continue to some degree with the aid of the IAVI.

The decision comes as a result of the unsatisfactory results of Merck's trial testing of its vaccines, which the company reported as not generating a robust enough immune response. For comparison, the immune responses of patients who had recovered from COVID-19 tended to have stronger immune responses than the vaccine, as well as competing vaccines. Merck's vaccines aren't the only two that have struggled during development and likely won't be the last due to the inherent difficulties of developing an effective vaccine.

"Merck's announcement in many ways underscores just how extraordinary the success of the Pfizer (PFE  ) and Moderna (MRNA  ) vaccines are. Merck's announced 'failure' comes on the heels of other reports on field trials of the Chinese and Russian vaccines that show modest - at best - efficacy," Dennis Carroll, former head of the Agency for International Development pandemic unit, told USA Today.

Merck's COVID medicines, which are now receiving the company's full attention, are still undergoing testing, with results expected after the trial ends in May. The drug, Molnupiravir, is being developed as part of a collaboration with Ridgeback Biotherapeutics.

There didn't appear to be too much of a reaction to Merck's announcement at the market (or if there was, it was very muted), with Merck's share price continuing the slight downward trend that it seems to have been stuck in for just under two weeks. On Monday, despite the announcement, Merck ended the day slightly up from Friday's close, ending Monday at $81.14, 0.20% up from Friday's closing price of $80.97. Merck lost those gains on Tuesday, ending the day's trading at $80.25, 1.1% down from Monday's close.