While he noted that the vaccine is currently optional and administered under the U.S. Food and Drug Administration's Emergency Use Authorization, full FDA approval could mean making the vaccine mandatory.
"Should the FDA approve it, then I am certain that Pentagon leadership will take a look at what our options are going forward, including the potential option of making mandatory," Kirby told a press briefing on Tuesday.
"It [the vaccine] is not FDA-approved and therefore it is still a voluntary vaccine," he added. "I would like to add that as we speak almost 69 percent of DoD personnel have received at least one dose. That's not bad."
Last week, the website Army Times reported that the U.S. Army has directed commands to prepare to administer mandatory COVID-19 vaccines beginning on Sept. 1, pending an expected full FDA approval.
The directive was an executive order by Department of the Army Headquarters.
In May, manufacturers Pfizer and BioNTech requested full approval of their vaccines by the FDA, with manufacturer Moderna seeking similar approval on June 1.
While the FDA has not responded, a May survey of civilians by the Kaiser Family Foundation indicated that one-third of unvaccinated adults would be more likely to obtain the vaccine if it is fully approved.
Over 930,000 U.S. service members have been fully vaccinated thus far.
Over 300,000 service members, dependents, civilian workers and contractors have been infected, leading to 357 deaths, a Defense Department update on Wednesday said.