How long does immunity from the COVID-19 vaccines last?

Three COVID-19 vaccines have already been approved for use in the United States, while several more are in development or available in other parts of the world.

As the rollout continues, what's become apparent in the past few months is that vaccines will play a prominent role in restarting travel and reopening the world. But one question readers have often posed still needs to be answered: How long does the immunity from the COVID-19 vaccines last? And how will that affect travel?

Here's what we know (and don't) about the COVID-19 vaccine and immunity and what it means for your travel plans.

How long does immunity from the COVID-19 vaccines last?

There are many things scientists still don't know about the COVID-19 vaccines, including how long immunity lasts.

"We currently do not know, as we are living through the experiment," said Dr. Jenny Yu, the senior manager of medical integrity at Healthline (which is owned by Red Ventures, TPG's parent company). "As time passes with the first group of people having received the vaccines as part of the clinical trial, their antibody levels will inform us how long this immunity will last."

What we do know is that all of the vaccines are effective in preventing COVID-19. According to the U.S. Centers for Disease Control and Prevention (CDC), The Pfizer-BioNTech vaccine was shown to be about 95% effective at preventing the COVID-19 disease in clinical trials, while the Moderna vaccine was 94.1% and Johnson & Johnson (Janssen) was 66.3%.

"We will know [in the future] whether we will need boosters or if it will behave more like the annual flu vaccines," Dr. Yu said.

Related: What you need to know about all the COVID-19 vaccines in the U.S.

Can you skip quarantine with a vaccine?

It's still early in the vaccine rollout process, and some countries have handled the rollout better than others. Still, several nations have already announced they are (or will soon) allow access -- and waive quarantine and testing mandates -- to fully vaccinated travelers.

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The CDC says that people are "fully protected" two weeks after their second dose of the Pfizer-BioNTech or Moderna vaccine or 28 days after the single-dose Johnson & Johnson vaccine.

Several U.S. states have adopted guidelines that allow people who have been fully vaccinated to skip quarantine:

• New Hampshire: Travelers do not have to quarantine or get tested if they are fully vaccinated, and it's been more than 14 days since the final vaccine, or if they tested positive in the last 90 days.
• Oregon: Oregon's policy doesn't specifically address travelers. However, people with exposure to COVID-19 do not have to quarantine if they are fully vaccinated, and it's been two weeks since the final dose and they've remained asymptomatic.
• Rhode Island: Travelers do not have to quarantine if they've received a final dose of a vaccine at least 14 days before their last exposure or out-of-state travel and it's been less than 90 days since their final vaccine dose and they haven't had symptoms since their last exposure or out-of-state travel.
• Vermont: People don't have to get tested for COVID-19 or quarantine if they've been in close contact with someone who tested positive as long as they do not have symptoms and are not in a health care setting.
• New York: New York State will ease entry and quarantine requirements for domestic travelers who have been fully vaccinated against COVID-19 within 90 days of travel.

Worldwide, several countries have also released guidelines. The Seychelles archipelago in the Indian Ocean is open to travelers from all countries who have been fully vaccinated, while Thailand plans to allow non-Thai travelers who have been vaccinated into the country in the second half of 2021.

Several European countries have indicated that they will waive quarantines for travelers who have completed the entire immunization cycle more than 14 days before the date of travel.

Unfortunately for travelers from the U.S., however, several of these countries, including Romania and Poland, will likely remain off-limits. Borders have remained closed for nearly a year now, a development that seems likely to continue as Europe and the U.S. battle new, aggressive variants of COVID-19.

Related: These European countries are allowing entry with vaccination

There's a time limit

Some destinations are imposing time limits on vaccinated travelers or those who have recovered from COVID-19. Already, we're seeing some states adopt this model as well.

If you're visiting New York state from elsewhere in the country, for example, you'll still need to test and quarantine if it's been longer than three months since you were vaccinated. And the guidelines don't apply to international travelers, either. This policy differs from CDC guidelines that say travelers should quarantine for seven days, even if they've been vaccinated.

Estonia says it will allow travelers who have recovered from COVID-19 or been vaccinated to enter the country -- but only if it's been no more than six months since recovery or vaccination. However, Estonia is still closed to Americans because of European Union restrictions.

And borders have remained closed with Europe from the U.S. for nearly a year now, a development that seems likely to continue as Europe and the U.S. battle new, aggressive variants of COVID-19.

Bottom line

More than a year after quarantines began in the U.S., more than 33 million Americans (about 10% of the population) have been fully vaccinated against COVID-19. But COVID-19 has proven to be an unpredictable disease, and efforts to stamp it out, such as vaccines, will take time to fully understand.

Now the question turns to how long immunity lasts, and how destinations will use that data to inform quarantine requirements as travel starts to reopen.