Members of the US military will be required to get the COVID-19 vaccine beginning next month under a plan laid out by the Pentagon and endorsed by President Joe Biden. In memos distributed to all troops, top Pentagon leaders said the vaccine is a necessary step to maintain military readiness.

Defense Secretary Lloyd Austin said the mid-September deadline could be accelerated if the vaccine receives final FDA approval or infection rates continue to rise.

I will seek the president’s approval to make the vaccines mandatory no later than mid-September, or immediately upon “licensure by the Food and Drug Administration” whichever comes first, Austin said in his memo sent Monday, warning them to prepare for the requirement.

The Pentagon plan provides time for the FDA to give final approval to the Pfizer vaccine, which is expected early next month. Without that formal approval, Austin needs a waiver from Biden to make the shots mandatory, and Biden has already made clear he supports it.

Austin’s decision reflects similar moves by governments and companies around the world, as nations struggle with the highly contagious delta variant that has sent new U.S. cases, hospitalizations, and deaths surging to heights not seen since last winter. The concerns are especially acute in the military, where service members live and work closely together in barracks and on ships, increasing the risks of rapid spreading. Any large virus outbreak in the military could affect America’s ability to defend itself in any security crisis.

Austin warned that if infection rates rise and potentially affect military readiness, I will not hesitate to act sooner or recommend a different course to the President if l feel the need to do so. To defend this Nation, we need a healthy and ready force.

In a statement Monday, Biden said he strongly supports Austin’s message to the force and the plan to add the COVID vaccine “to the list of required vaccinations for our service members not later than mid-September.

Biden said the country is still on a wartime footing and being vaccinated will enable our service members to stay healthy, to better protect their families, and to ensure that our force is ready to operate anywhere in the world.

Austin’s memo, which went out Monday, was followed quickly by one from Army Gen. Mark Milley, chairman of the Joint Chiefs of Staff.

“The Secretary of Defense intends to mandate vaccinations for all Service members in the coming weeks”, said Milley, adding that the military’s medical professionals recommended the move. At the bottom of his message, Milley scrawled a handwritten note: Getting vaccinated against COVID-19 is a key force protection and readiness issue.

The decision comes a bit more than a week after Biden told defense officials to develop a plan requiring troops to get shots as part of a broader campaign to increase vaccinations in the federal workforce.

More broadly, the COVID-19 crisis has worsened around the country, with hospitals experiencing deeper strain in unvaccinated areas of the South. Mississippi reported that 35 medical centers are completely out of intensive care unit beds, Arkansas topped its pandemic record for COVID admissions, and the average number of people hospitalized nationwide has returned to levels not seen since February. More patients are being parked in emergency rooms while they wait for beds to open up and the average number of daily deaths is now above 500.

The country is averaging about 108,000 new infections and 700,000 vaccines administered a day.

Austin said the military services will have the next few weeks to prepare, determine how many vaccines they need, and how this mandate will be implemented. The additional time, however, also is a nod to the bitter political divide over the vaccine and the knowledge that making it mandatory will likely trigger opposition from vaccine opponents across state and federal governments, Congress, and the American population.

Pentagon spokesman John Kirby said Monday that he believes the military has enough vaccines to meet the requirements. He added You can consider this memo not just a warning order to the services but to the troops themselves.

Democratic and Republican leaders of the House Armed Services Committee said vaccines have proven effective.

Some may try and criticize the Secretary’s decision, using anti-vax arguments that are not supported by facts or science to politicize the conversation. These desperate attention seekers must be ignored, said Rep. Adam Smith, D-Wash., chairman of the House Armed Services Committee. Rep. Mike Rogers, R-Ala., said the vaccine will help protect troops who live in cramped conditions and don’t have the option to telework.

Rep. Mark Green, R-Tenn., however, said Austin should not mandate a vaccine that doesn’t have full FDA approval. Wearing our country’s uniform does not mean our service members sign away the right to make personal medical decisions,” he said.

The decision will add the COVID-19 vaccine to a list of other inoculations that service members are already required to get. Depending on their location, service members can get as many as 17 different vaccines.

Austin’s memo also said that in the meantime, the Pentagon will comply with Biden’s order for additional restrictions on unvaccinated federal personnel, including masks, social distancing, and travel limits.

According to the Pentagon, more than 1 million troops are fully vaccinated and another 237,000 have received one shot. But the military services vary widely in their vaccination rates.

The Navy said that more than 74% of all active duty and reserve sailors have been vaccinated with at least one shot. The Air Force, meanwhile, said that more than 65% of its active-duty and 60% reserve forces are at least partially vaccinated, and the number for the Army appears closer to 50%.

Military officials have said the pace of vaccines has been growing across the force, with some units such as sailors deploying on a warship seeing nearly 100% of their members get shots. But the totals drop off dramatically, including among the National Guard and Reserve, who are much more difficult to track.

Some unvaccinated troops have said they’d get the shot once it’s required, but others are flatly opposed. Once the vaccine is mandated, a refusal could constitute a failure to obey an order and may be punishable under the Uniform Code of Military Justice.

Army guidance, for example, includes counseling soldiers to ensure they understand the purpose of the vaccine and the threat the disease poses. The Army also notes that if a soldier “fails to comply with a lawful order to receive a mandatory vaccine, and does not have an approved exemption, a commander may take appropriate disciplinary action.

Military service officials say they don’t have data on the number of troops who have refused other mandated vaccines, such as anthrax, chickenpox, or flu shots over the past decade or more. And they weren’t able to provide details on the punishments service members received as a result of the refusal.

Officials said they believe few troops have refused other mandated vaccines, and the discipline can vary.

Also, service members can seek an exemption from any vaccine either temporary or permanent for a variety of reasons including health issues or religious beliefs. Regulations say, for example, that anyone who had a severe adverse reaction to the vaccine can be exempt, and those who are pregnant or have other conditions can postpone a shot.

Navy officials said last week that there has been only one case of COVID-19 hospitalization among fully vaccinated sailors and Marines. But, the Navy said there have been more than 123 hospitalizations in a similar group of unvaccinated sailors and Marines. It said fewer than 3% of its immunized troops have tested positive for COVID-19.

The other military services did not provide similar data.