The Obama administration on Wednesday pushed back against congressional criticism that available funding to combat the Zika virus is not being spent fast enough, claiming all the money on hand for domestic Zika efforts will be exhausted by the end of September.  

In a letter to senior health and foreign appropriators in both chambers, Sylvia Mathews Burwell, the secretary of Health and Human Services, provided further details of how the department is spending its share of the $589 million that the administration reprogrammed in April to fight Zika. Roll Call has reported recently that much of that total has yet to be spent, according to information from the Office of Management and Budget.  

Burwell’s letter also highlights the need for additional funding to be provided by Congress, a case the administration has struggled to make while Republicans have accused Obama of sitting on the initial batch of money. A conference report that includes additional Zika response funds is stalled in Congress.  

Available funding for the major HHS agencies involved in the Zika response will be fully spent by the Sept. 30 end of fiscal 2016 or sooner, Burwell wrote, and a funding lapse at that point would put crucial anti-Zika efforts on hold.  

“The department is committed to using scarce federal dollars aggressively and prudently, especially in light of Congress’ inaction to provide any additional resources and the uncertainty around whether Congress will provide resources in the future,” she said, referring to the partisan gridlock that has so far prevented lawmakers from sending the Zika spending legislation to the White House.  

The $374 million that went to HHS for domestic purposes was split among the Centers for Disease Control and Prevention, the National Institutes of Health and the Biomedical Advanced Research and Development Authority. CDC received $222 million, the bulk of the money, since the agency is “on the front lines of providing assistance to states, territories and localities to fight Zika,” she said.  

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As of the beginning of August, Burwell said, CDC had obligated $123 million of the domestic total, and the agency announced Tuesday that it was doling out an additional $16 million in state awards. The rest of the CDC funds will be obligated by the end of the fiscal year, Burwell added.  

The remaining CDC domestic funds will be used for deploying emergency response teams, testing Zika specimens, providing grants to states and organizations to battle mosquitoes, studying the health effects of the virus and more.  

The CDC also received a separate $78 million in repurposed funding to be used exclusively for international response efforts, bringing the HHS grand total to $452 million. The remaining $137 million went to the U.S. Agency for International Development, according to an OMB aide.  

Burwell also detailed several activities that would be curtailed if Congress continues withholding additional dollars.  

CDC mosquito control and surveillance efforts in the U.S. will be “severely limited,” Burwell said, along with maternal and child health services provided by the Health Resources and Services Administration. And the Centers for Medicare and Medicaid Services wouldn’t have authority to provide additional federal matching funds to Puerto Rico and other territories without further congressional action, she added.  

Efforts to test Zika vaccines at NIH could stall when the agency exhausts the $47 million it received, which NIH projects will occur at the end of August. The agency has started initial trials for a DNA-based Zika vaccine, Burwell said, but it won’t be able to start the second phase of trials without additional funding, meaning it will take longer to make vaccines available to the public.  

An NIH study of infants and pregnant women with Zika would also be delayed.  

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“Additional funding is needed to accelerate and expand enrollment in [the study], and continue following the infants through their first year to provide critical answers regarding the range and true risk of congenital anomalies caused by the virus,” Burwell wrote.  

The Biomedical Advanced Research and Development Authority, or BARDA, which distributes funding to private sector partners to develop medicines and vaccines, also estimates that it will exhaust its $85 million in Zika funding by the end of the month. That could hamper efforts to develop diagnostic tests and other needs.  

CDC, meanwhile, is on pace to exhaust its $222 million in domestic Zika funding by Sept. 30, the end of the fiscal year.  

“In regards to the immediate needs that NIH and BARDA face, I am evaluating all options in order to avoid delaying the development of a Zika vaccine,” Burwell said. “At the same time, I want to urge you to work with your leadership to develop a bipartisan bill that will allow us to mount the full and timely response to the Zika virus that the American people deserve.”  

On Wednesday, NIH announced that it had begun administering its experimental Zika vaccine to human subjects, beginning the first phase of its trial to determine the vaccine’s safety a month earlier than it had originally expected. Another vaccine from a private pharmaceutical company began similar testing last week.  

With Congress out of town for a seven-week summer recess ending Sept. 6, lawmakers — particularly those up for re-election — continue to feel the political heat as the Zika situation worsens. Sen. Charles E. Grassley sent out an email Wednesday defending his vote for a bipartisan $1.1 billion compromise measure included in a broader appropriations package.  

“There’s an effort to distort my votes on spending to fight the Zika virus to make it seem as if I don’t care about women and children’s health,” the Iowa Republican wrote. “That’s not the case.”  

Grassley said he’s prodded the Obama administration to act more quickly to disburse the repurposed funding. He criticized Senate Democrats for blocking a House-passed conference report over objections to offsets and other language related to contraception and environmental protections.  

Republican Sen. Marco Rubio, seeking reelection in Florida, has also focused heavily on Zika since ending his GOP presidential bid and returning to the Senate. On Tuesday, Rubio said he spoke with Senate Majority Leader Mitch McConnell of Kentucky about lobbying House Republican leaders to compromise on a new Zika deal.  

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House members from Florida are also sounding the alarm, pushing the CDC to give Florida a larger slice of grant funding than the state has received so far.  

In a letter signed by every member of the Florida delegation, Republican Rep. Ileana Ros-Lehtinen said the $720,000 the state received out of $16 million in new CDC grants was a “paltry sum,” amounting to about 4 percent of the grants despite the urgent needs particular to Florida.  

Andrew Siddons contributed to this report .