Dr. Anthony Fauci stressed Wednesday that the speed of vaccine development “was not compromising safety, nor was it compromising scientific integrity.”

"The speed is really a reflection of the extraordinary scientific advances that have been made that allowed us to do things in weeks to months that normally would have taken several years," Fauci explained at the Johns Hopkins Bloomberg School of Public Health Summit. "The process that went into deciding the safety and efficacy was both independent and transparent."

"We've got to be doing a lot of community engagement and community outreach to get people to understand," Fauci said.

ABC News' Sophie Tatum contributed to this report.

An 83-year-old passenger on board Royal Caribbean International’s Quantum of the Seas tested positive for COVID-19 on Wednesday, Singapore's Ministry of Health said in a statement.

The passenger had reported to the ship medical center with symptoms and was tested with PCR equipment on board. The 83-year-old was immediately isolated after getting the results.

Close contacts of the individual have tested negative and have been quarantined.

Singapore's National Public Health Laboratory will re-test that sample and a second sample will be taken to verify the results.

An epidemiological investigation has been initiated and contact tracing is in progress, the Ministry of Health said. All passengers will undergo mandatory COVID-19 testing before they are allowed to leave the terminal at Marina Bay Cruise Centre.

The ship will go through a "deep cleaning and disinfection," the cruise line said in a statement.

"The health and safety of our guests and crew are of utmost importance to us, and we have made the decision to cancel the 4-night cruise departing tomorrow, December 10," Royal Caribbean said.

ABC News' Christine Theodorou contributed to this report.

Authorities in the U.K. were investigating reports of two allergic reactions to the coronavirus vaccine from Pfizer/BioNTech -- a day after the country started its historic inoculation program.

Dr. June Raine, head of the UK regulatory body Medicines and Healthcare products Regulatory Agency (MHRA), said the agency was looking at two reports of allergic reactions to the vaccine, which rolled out yesterday.

"We know from the very extensive clinical trials that this wasn’t a feature but if we need to strengthen our advice... we get that advice to the field immediately,” said Raine.

The vaccine is believed to be both safe and 95% effective, according to Pfizer/BioNTech. The U.S. Food and Drug Administration was set to hold a hearing for emergency use of the drug on Thursday.

“There have been two cases of anaphylactoid reactions in individuals with a strong past history of allergic reactions both of whom carried an adrenaline auto injector,” the National Health Service (NHS) said in a statement on Wednesday. “These individuals developed symptoms of anaphylactoid reaction shortly after receiving the vaccine. Both recovered after appropriate treatment. We are seeking further information and will issue further advice following investigation.”

Pfizer/BioNTech have not yet commented on these latest findings.

Professor Stephen Powis, national Medical Director for the NHS, said: “As is common with new vaccines the MHRA have advised on a precautionary basis that people with a significant history of allergic reactions do not receive this vaccination after two people with a history of significant allergic reactions responded adversely yesterday. Both are recovering well.”

Russia registered 26,190 new coronavirus infections and 559 fatalities linked with the virus over the past 24 hours, officials said on Wednesday.

In Moscow, 5,145 new infections and 75 fatalities have been confirmed over the same time, according to official information.

The all-time high of 29,000 new cases was registered just last weekend but since then the number has been declining.

Since the start of the pandemic, Russia has confirmed 2,541,199 coronavirus infections and 44,718 deaths, officials have said.

ABC News' Alina Lobzina contributed to this report.

U.S. Surgeon General Jerome Adams urged people to take the coronavirus pandemic seriously and follow public health guidelines as infections and hospitalizations soar across the country.

"I want the American people to know this virus is incredibly unforgiving, cases are going up, hospitalizations are going up, my colleagues are dog-tired and we need you to hang on just a little bit longer because we've got vaccines coming but we want as many people to be alive to get them as possible and a lot of that is going to depend on your behavior," Adams told ABC News chief anchor George Stephanopoulos in an interview Monday on "Good Morning America."

Although "more people than ever are wearing masks," Adams said he's "very" concerned by the number of people who still "don't understand how much spread is occurring by people who don't have symptoms."

About 20 million "full doses" of COVID-19 vaccines will be available by the end of the year, according to Adams, who urged every American to be immunized against the virus as soon as a vaccine is authorized and made available.

"It's a way that we can ultimately end this pandemic, but it doesn't matter if people won't get the vaccination," the surgeon general said. "We know that vaccine levels are only about 50% for adults for flu and they go down to about 40% for African-Americans."

Adams said he is working with historically black colleges and universities as well as faith communities to bolster vaccine confidence among all populations. The White House is also hosting a vaccine summit Tuesday, he said.

"One thing you can all do right now, it's national flu immunization week, get your flu shot because half a million people were hospitalized last year with the flu," Adams said. "We simply can't afford for that to happen this year with hospitals being overwhelmed."

The surgeon general said the current surge in infections across the country "is different than earlier surges," because it's not about a lack of masks or personal protective equipment, nor is it due to a lack of testing.

"It's really about health care capacity, and certain places are just being overwhelmed," he said. "So we know that we can actually help them with their health care capacity by immunizing their health care staff. We're going to leave it up to the states, but we're going to give them guidance."

Adams noted it's also important to vaccinate those who are most likely to die from COVID-19.

"We know that 40 to 50% of the deaths are occurring in people who are in longterm care facilities who are older," he said.