Given all the problems faced by the cruise industry in 2020, the announcement by one operator that all passengers must be fully vaccinated for COVID-19 before they board sounds sensible.

But when Saga Cruises this week became the first to introduce the requirement, not everyone agreed -- the British operator was inundated with so many responses, good and bad, it temporarily made its Twitter account private while it dealt with them.

Nevertheless, the move by Saga will be eagerly watched by many in the cruise industry interested to see if whether a vaccine rule will help kick start travel, or prove as divisive as other attempts to work around COVID-19.

Saga, which caters mostly to British people over the age of 50, told CNN Travel that the vaccinated-passengers-only rule was prompted by the results of a recent customer poll, which suggested 95 per cent of regular Saga customers would support such a policy change.

Saga Holidays' CEO Chris Simmonds said in a statement that the decision was made partly because "many of our customers [are] amongst the first groups of people to be offered the vaccine."

"With this in mind and having spoken with our customers, we want to ensure we are providing the safest possible experience whilst they are on holiday with us," said Simmonds.

As well as offering cruises, Saga also organizes tours and all-inclusive holidays. Travelers will also need to be vaccinated before embarking on these vacations.

Before boarding a Saga ship, passengers must have had both COVID-19 jabs at least 14 days before departure.

Right now, Saga's operations are paused, but the company aims to restart cruises in May 2021. If the U.K.'s vaccine rollout goes to plan, by then, a substantial number of Brits over 50 should have been inoculated against coronavirus.

Saga said crew, who largely skew younger, would not need to be vaccinated before working on board, stating that other protocols would be in place to protect staff until they're able to receive inoculation.

The announcement raises the question of whether compulsory pre-boarding Covid vaccinations could become the norm for cruise passengers and/or staff.

Previously, testing had previously been championed as the key to unlocking the industry, but when seven people tested positive on-board small cruise ship SeaDream 1 in Nov. 2020, the efficacy of preboarding testing was called into question.

'MULTI-LAYERED APPROACH'

Cruise Line International Association (CLIA), a global body that represents 95 per cent of the world's cruise fleet, said "a multi layered approach" to on-board safety "is the right one to mitigate risk."

Bari Golin-Blaugrund, a spokesperson for CLIA, said measures implemented by cruise lines are being constantly evaluated and will evolve "as the pandemic and circumstances change over time."

Golin-Blaugrund wouldn't comment on whether CLIA would enforce a vaccinated passengers-only rule for its member cruise lines.

"We share in the excitement surrounding the development of a vaccine for COVID-19 and are optimistic that it will help facilitate the global recovery from the pandemic," she told CNN Travel.

"At the same time, we understand that the rollout of the vaccine will take some time."

Saga confirmed to CNN Travel that it will still instigate other safety measures on board its sailings, including reduced capacity, pre-departure COVID-19 testing, social distancing, increasing cabin air flow and improving on-board medical facilities.

British cruise goer Sara Roberts, 59, who was a passenger on board the virus-hit Coral Princess back in spring 2020, told CNN Travel she thinks a widespread return to cruising will only happen if and when passengers are confident ships are Covid safe.

"Vaccination is a good way forward, providing the vaccine is proven to work," said Roberts, who expressed concern at the current wait times in the U.K. between administration of the first dose and second dose.

Following her experience last year, Roberts also has lingering concerns surrounding cruise travel more generally.

"We have sailed with the majority of cruise lines over the last 15 years and it had always been my preferred type of holiday," she said.

"However our experience aboard the Coral Princess made me realize you are not in control of your own destiny whilst aboard a cruise. Therefore, I would not consider embarking another for the foreseeable future until COVID is no longer a risk."

GLOBAL CRUISE LINE PERSPECTIVE

For cruise lines that serve passengers from across the world, and of all ages, establishing a vaccinated-passenger-only policy could be difficult.

But alongside Saga's announcement, there is already some precedent for this. Australian airline Qantas last year said passengers will need to be vaccinated before boarding international flights. Meanwhile Singapore Airlines recently announced plans to become the world's first fully vaccinated airline, pledging to inoculate all crew and staff.

There's also a possibility that port cities will only allow travelers to disembark cruise ships if they have proof of vaccination, which would take the decision out of the cruise lines' hands.

Last fall, when the U.S. Centers for Disease Control and Prevention's ban on cruising in U.S. waters was lifted, the CDC introduced its Framework for Conditional Sailing Order for cruise ships, outlining the lengthy process cruise lines need to follow to recommence US cruising.

Measures include mock "trial" cruises, universal mask wearing, physical distancing and COVID-19 testing. There's no mention of compulsory vaccinations. The guidance was issued before the vaccines had been approved.

When asked whether Royal Caribbean, which owns Royal Caribbean Cruise Line alongside Celebrity Cruises and Silversea Cruises, would adopt a vaccinated-passengers-only policy, spokesperson Jonathon Fishman told CNN Travel that the company was "still in the process of finalizing the details for our return to service."

"As soon as we have more information on our requirements, we will let our guests know," he added.

MSC Cruises, which was one of the first major cruise lines to restart operations last summer -- in the form of a seven-day, Italian-residents-only, Mediterranean cruise -- declined to comment, deferring to the CLIA.

MSC's voyages were paused over the festive season due to the new Italian lockdown, but MSC Grandiosa is due to restart Italian voyages this weekend.

U.K.-based cruise line Fred Olsen said it had no news to share on this front as yet.

Roger Frizzell, who represents Carnival Corporation -- the cruise giant that owns Carnival Cruise Line, Costa Cruises, Princess Cruises, Cunard, Holland America and P&O Cruises -- also said no firm decisions had been made.

"The new vaccines represent an important breakthrough for people throughout the world, including the travel, hospitality and cruising industries," said Frizzell.

"We are reviewing the various vaccines, but we have not made any decisions on next steps at this point."

Meanwhile, Norwegian Cruise Line stated that "all options regarding vaccinations" were being explored for guests and crew -- but that staff safety would be at the fore.

"It is our intention that all crew members be vaccinated before boarding our vessels to begin their duties, subject to availability of the vaccine," said a Norwegian spokesperson.

CREW PERSPECTIVE

Conny Seidler, who worked as a dancer on board the Costa Deliziosa during the first half of 2020, said she'd long expected pre-boarding COVID vaccinations to become compulsory for crew.

The Deliziosa was the last ship carrying large numbers of passengers to make it back to port amid the global shutdown of the cruise industry last year.

"You need certain vaccines to be able to work on a cruise anyway," points out Seidler, citing the yellow fever jab and tuberculosis as examples.

"From the point of view of the cruise, it's obviously safer and lower risk if all the crew is vaccinated."

Seidler, who is from Austria, acknowledges that such a regulation could put off some people, but she thinks most crew members would welcome this rule.

Many crew are currently out of work, and experienced a tough time working on COVID-hit vessels in the wake of the pandemic.

"I honestly don't think there's going to be a lot of crew members who are going to be against the vaccine," says Seidler. "I can imagine most of the crew will be like: 'As long as I can work, I'm happy to do it.'"

As for the passengers, Seidler suggests some may be unwilling to travel unless they know everyone on board has been vaccinated.

But Seilder thinks most big cruise lines will be hesitant to introduce a vaccinated-passengers-only rule, because the global vaccine rollout won't be fast enough, and it could deter certain guests.

Still, Seidler reckons the more people vaccinated on board, the safer the environment will be, and the likelier it is that cruising can recommence successfully.