In a blow to hopes that cruises will sail sooner rather than later, over the course of just five days this month, four lines that resumed service reported Covid-19 cases on their ships.

They included a passenger on an UnCruise Adventures Alaska sailing, the first cruise in U.S. waters since March; a Hurtigruten outbreak that had affected 36 crew members as of Aug. 4; an American guest on the Tahiti-based Paul Gauguin's first cruise with international passengers; and a report from SeaDream Yacht Club that a Danish passenger on one of its two ships tested positive.

Except for Hurtigruten, which quickly admitted it had made mistakes and did not follow its own or Norway's policies, the cases were very similar: asymptomatic passengers were detected by testing mandated by the municipality the ship was in. With UnCruise and Paul Gauguin, all passengers had tested negative only days before the positive result.

The incidents raised questions about cruise lines' ability to start sailing at this point in the Covid-19 pandemic.

UnCruise CEO Dan Blanchard said that, absent a vaccine, only reliable, rapid testing can change the situation.

"And trust me, we tried," he said.

UnCruise had felt confident that the nature of its trip would allow the line to create "a bubble," with crew quarantined more than a month onboard before the first cruise and no stops at ports. The only "misfire," he said, was the testing.

All passengers wore masks and social distanced onboard. Alaska requires a test upon arrival, which is how the infection was detected, but the results weren't delivered until after the passenger boarded and the ship sailed.

UnCruise had submitted a plan of action to Alaska in the event a positive case was reported, and it was unfolding accordingly.

Paul Gauguin Cruises had said that, as of last week, the negative results of everyone else tested on the ship, "confirms the quality of the health protocols in force onboard."

Some experts said the situations were part of the industry's learning curve.

Dr. Jewel Mullen, an associate dean at University of Texas Austin's Dell Medical School, said that rather than calling the cases "a complete failure, this is part of what we need to know."

Dr. Jewel Mullen

"What we are now observing [will help us] understand how to continue to refine all of the preventive and protective measures that this industry can undertake," she said. "I have never heard anybody say that once travel resumes, there won't be any cases. Testing is a key part of trying to be as proactive as possible, one of the components of getting this right and making this as safe as possible.

"It's not the only component and will not be the ultimate solution. It will be a part of what needs to happen."However, news of the outbreaks prompted former Windstar president John Delaney to say that cruise lines should stay on pause until there is a vaccine.

"I think the traveling public won't feel comfortable or confident cruising again until that time," he said.

"The industry should just say, 'it looks like there will be a vaccine by early 2021, let's just agree we're not going to sail until then.' That's an obligation the industry should have to their crew and also to the ports that they visit."

Brad Tolkin, co-CEO of World Travel Holdings, said that the recent events had not created any change in consumer demand but added that, "in general, the consumer is not ready to travel."

"As we have witnessed in Major League Baseball and in many states in the U.S., the virus is still out there," he said. "As such, the consumer probably expected an outbreak. It would be naive to think that there would not be one."

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Positive tests on small ship lines raise the question: Too soon? - Travel Weekly