The COVID-19 Delta variant has been sweeping across the country, reminding us that the pandemic remains a very present danger.

COVID-19 regulations have fluctuated over the year, to say the least. At the start of the pandemic, we were all learning about what we could and couldn’t do, and now, while some follow guidances more closely than others, we know the basics: face masks protect against illness, especially when people use them in places that are crowded or poorly ventilated.

While there are many preventative measures you can take, some situations are just a recipe for the spread of the COVID-19 Delta variant, at least according to one expert.

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Luis Ortofsky, MD, explained to CNBC, that traveling aboard cruise ships is among the most risky situations you can get yourself into right now. “It’s just a recipe for transmission,” he said. While all sorts of travel while unvaccinated is currently discouraged, a cruise is much riskier than a flight, where people wear their masks for the duration of it and there’s presumably good airflow.

Ortofsky makes it clear that while being vaccinated is a good thing, with the Delta variant in circulation, it “no longer guarantees that you’re not going to acquire the infection, or be able to transmit it,” he said. We can try to do cruises as safely as possible, but we are going to have breakthrough cases.”

Another complication that’s thrown in the mix are state laws, like Texas, banning state and local mandates that require COVID-19 vaccinations, making it more difficult for cruises to protect their passengers and employees.

Cruises are particularly tough places to avoid a virus. You eat and drink there, usually in indoor locations where other people are doing the same. The majority of your time is spent in common areas, interacting with people and touching a variety of surfaces, making it very easy for germs to transmit from person to person. It’s not only COVID-19 that’s easy to spread; cruises are notorious for having respiratory and gastrointestinal viruses spreading from passenger to passenger.

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It could be a while before you are able to board a cruise safely. Still, the safest way to board one would be to ensure that the cruise line you’re traveling with asks for proof of vaccination and that allow for reduced capacity. While you may be able to dine outdoors, cruises have most of their entertainment in indoor locations, so a degree of risk is implicit.