The US does not have hard-and-fast rules when it comes to American travelers being vaccinated for the flu abroad.
“People should go in their local clinics and schools, and be vaccinated,” Dr. Michelle Smith, a spokeswoman for the Centers for Disease Control and Prevention (CDC), said in March.
“They should stay in contact with their own doctors so that they don’t have to travel [for care] if they don’t feel well,” she added.
On Thursday, the agency announced that it is allowing American travelers who have been vaccinated against the flu in countries where flu season is not circulating (such as Australia) to travel to the US — without receiving shots again for the US variant of the flu.
It’s been an unusually busy flu season in the US, where the H3N2 strain of the virus has been circulating more frequently. The US has had 14,322 flu-related hospitalizations and 2,158 deaths since October 1, according to the CDC, which tracks the illness.
It is not unusual for the CDC to allow travelers to enter the US who have been vaccinated against the flu, provided the H3N2 variant is not circulating in the US. For example, in 2016, the CDC allowed travelers to travel to the US who had been vaccinated against the H3N2 strain of the flu.
However, it is not uncommon for travelers to need a second or subsequent dose of the vaccine in countries where the virus is not circulating. This strain accounts for about 55% of flu illnesses and complications in the US. Travelers diagnosed with the H3N2 strain can have the flu season to follow, along with some or all of their visits to an American doctor, the CDC has previously said.
“We have not changed the national public health recommendations for influenza vaccine or ongoing ongoing recommendations for influenza vaccination for people in the US,” Smith said in a statement from the agency on Thursday.
“When one virus strain of influenza shows up in multiple regions, as was the case this year, it is reasonable to allow people to travel to the US with the same immunity they received while they were traveling to another flu outbreak zone,” she said.
“However, the CDC always recommends people be vaccinated for the season they are visiting. Since most people who are able to travel will not need a second dose, travel agencies that market influenza vaccinations typically will recommend a second dose.”
However, the CDC says that regardless of which seasonal flu season a traveler visits, they should seek influenza vaccination at least a week before leaving their home country.
This includes secondary infections, which can occur with the flu after a person has been vaccinated, but before returning to their home country. “A traveler to an outbreak zone and someone with a primary vaccination or secondary vaccination has the same immunity but can develop secondary infections that occurred before vaccination,” the CDC says.