WASHINGTON — A huge inter­na­tion­al study of a COVID-19 vac­cine that aims to work with just one dose is get­ting under­way as top U.S. health offi­cials sought Wednes­day to assure a skep­ti­cal Con­gress and pub­lic that they can trust any shots the gov­ern­ment ulti­mate­ly approves.

Hopes are high that answers about at least one of sev­er­al can­di­dates being test­ed in the U.S. could come by year’s end, maybe sooner.

“We feel cau­tious­ly opti­mistic that we will be able to have a safe and effec­tive vac­cine, although there is nev­er a guar­an­tee of that,” Dr. Antho­ny Fau­ci, infec­tious dis­ease chief at the Nation­al Insti­tutes of Health, told a Sen­ate committee.

Pres­i­dent Don­ald Trump is push­ing for a faster time­line, which many experts say is risky and may not allow for ade­quate test­ing. On Wednes­day he tweet­ed a link to news about the new John­son & John­son vac­cine study and said the Food and Drug Admin­is­tra­tion “must move quickly!”

“Pres­i­dent Trump is still try­ing to sab­o­tage the work of our sci­en­tists and pub­lic health experts for his own polit­i­cal ends,” Sen. Pat­ty Mur­ray, a Demo­c­rat from Wash­ing­ton state, said before tick­ing off exam­ples of pres­sure on the FDA.

FDA Com­mis­sion­er Stephen Hahn pledged that career sci­en­tists, not politi­cians, will decide whether any coro­n­avirus vac­cine meets clear­ly stat­ed stan­dards that it works and is safe. Vac­cine devel­op­ment usu­al­ly takes years but sci­en­tists have been rac­ing to short­en that time, in part by man­u­fac­tur­ing dos­es that will have to be thrown away if stud­ies find they don’t work.

“Sci­ence will guide our deci­sions. FDA will not per­mit any pres­sure from any­one to change that,” Hahn said. “I will put the inter­est of the Amer­i­can peo­ple above any­thing else.”

FDA has faced crit­i­cism for allow­ing emer­gency use of some COVID-19 treat­ments backed by lit­tle evi­dence, but Hahn said if vac­cine mak­ers want that faster path to mar­ket, addi­tion­al stan­dards will be com­ing soon.

As for the test­ing of vac­cine can­di­dates, Fau­ci added: “There is no cut­ting corners.”

A hand­ful of vac­cines already are in final test­ing in the U.S. and oth­er coun­tries. In one of the largest stud­ies yet, John­son & John­son aims to enroll 60,000 vol­un­teers to test its sin­gle-dose approach in the U.S., South Africa, Argenti­na, Brazil, Chile, Colom­bia, Mex­i­co and Peru. Oth­er can­di­dates in the U.S. require two shots.

J&J’s vac­cine is made with slight­ly dif­fer­ent tech­nol­o­gy than oth­ers in late-stage test­ing, mod­eled on an Ebo­la vac­cine the com­pa­ny created.

Final-stage test­ing of one exper­i­men­tal vac­cine, made by AstraZeneca and Oxford Uni­ver­si­ty, remains on hold in the U.S. as offi­cials exam­ine whether it pos­es a safe­ty risk.

Beyond vac­cines, Trump reg­u­lar­ly under­cuts con­fi­dence in his own pub­lic health agen­cies, such as false­ly tweet­ing about a ”deep state, or who­ev­er at FDA” — and in recent weeks, some polit­i­cal appointees were forced out after alle­ga­tions they inter­fered with sci­en­tif­ic advice.

Con­spir­a­cy the­o­ries are sap­ping the morale of dis­ease fight­ers work­ing 24/7 at the Cen­ters for Dis­ease Con­trol and Pre­ven­tion, Dr. Robert Red­field, its direc­tor, told the Sen­ate com­mit­tee on health, edu­ca­tion, labor and pensions.

“It’s offen­sive to me when I hear this type of com­ment,” said Red­field, not­ing that CDC, like the mil­i­tary, strives to be nonpartisan.

Yet Red­field strug­gled to defend against crit­i­cism that CDC bowed to polit­i­cal pres­sure with guide­lines that dis­cour­aged test­ing of peo­ple with­out COVID-19 symp­toms. Asymp­to­matic peo­ple do spread the virus and CDC, under fire, lat­er changed the guide­lines’ word­ing. Red­field insist­ed it all amount­ed to mis­in­ter­pre­ta­tion and stressed Wednes­day: “More tests will actu­al­ly lead to less cases.”

More than 200,000 Amer­i­cans have died from COVID-19 so far this year, and in many states, infec­tions still are climb­ing. The U.S. is con­firm­ing an aver­age of 41,968 new dai­ly cas­es, up 13% com­pared with the aver­age two weeks ago.

Fau­ci was blunt: More lives could have been saved if every­one in the coun­try bet­ter fol­lowed rec­om­men­da­tions to wear masks, avoid crowds and keep 6 feet apart.

“We know some states did a good job. Some states did not so good a job. Some states tried to do a good job but peo­ple didn’t lis­ten,” he said, sin­gling out mask-less crowds in bars. Going for­ward, “we need uni­for­mi­ty through­out the country.”

In a testy exchange, Sen. Rand Paul of Ken­tucky insist­ed pub­lic health offi­cials were wrong that a lock­down could change the course of the pan­dem­ic. A vis­i­bly angry Fau­ci accused the Repub­li­can of repeat­ed­ly mis­con­stru­ing his statements.

“I don’t regret say­ing that the only way we could have real­ly stopped the explo­sion of infec­tion was by essen­tial­ly — I want to say shut­ting down,” he said.

Fau­ci dis­missed Paul’s con­tention that hard-hit New York has become large­ly immune because so many peo­ple were infect­ed: “If you believe 22% is herd immu­ni­ty, I believe you’re alone in that.”

Fau­ci also called atten­tion to so-called “long-haulers,” COVID-19 sur­vivors who con­tin­ue to strug­gle with a range of symp­toms such as pain, fatigue, even heart dam­age. He warned much remains unknown about the long-term effects of the virus.

Democ­rats warned those sur­vivors are at risk of being denied insur­ance if the Trump admin­is­tra­tion suc­ceeds in over­turn­ing an Oba­ma-era health law that for­bids com­pa­nies from turn­ing down peo­ple with pre-exist­ing health prob­lems or charg­ing them more. The death of Jus­tice Ruth Bad­er Gins­burg means there are no longer five jus­tices on the Supreme Court who have upheld the Afford­able Care Act.

“We will see rates sky­rock­et for any­body who has had COVID,” pre­dict­ed Sen. Chris Mur­phy, D‑Conn.

A vac­cine “will go a giant step” in con­trol­ling infec­tion, but Fau­ci warned peo­ple still will need to take those pre­cau­tions for a while after the first vac­cine arrives because it won’t change con­di­tions overnight.

Why? It’s unusu­al for a vac­cine to be 100% effec­tive. There won’t be enough at first for every­one, and even once there is, it will take months to get the shots into the arms of every Amer­i­can who wants one — an effort CDC’s Red­field sees stretch­ing into June or July.

Because of the enor­mous logis­ti­cal chal­lenges, CDC wants states to get ready now and on Wednes­day, announced they would get $200 mil­lion to help begin set­ting up those operations.

FDA has faced crit­i­cism for allow­ing emer­gency use of some COVID-19 treat­ments backed by lit­tle evi­dence. Vac­cines, unlike ther­a­pies, are giv­en to healthy peo­ple and thus usu­al­ly require more proof.

FDA already has told man­u­fac­tur­ers it expects any COVID-19 vac­cine to be at least 50% effec­tive, and Hahn said addi­tion­al stan­dards are com­ing soon for any that decide to seek the faster emer­gency-use authorization.

Neer­gaard report­ed from Alexan­dria, Vir­ginia. AP Writer Lin­da John­son con­tributed from Fair­less Hills, Pennsylvania.

The Asso­ci­at­ed Press Health and Sci­ence Depart­ment receives sup­port from the Howard Hugh­es Med­ical Institute’s Depart­ment of Sci­ence Edu­ca­tion. The AP is sole­ly respon­si­ble for all content.

Sub­scribe to bi-week­ly newslet­ter to get health news sent straight to your inbox.