PCR Testing Program Finds Breakthrough COVID-19 Cases Among Vaccinated Passengers


A new report from a Qatari pilot program showed vaccines are highly effective against COVID-19, but they do not completely eliminate risk.


A new report based on PCR tests of airline passengers shows that even as international travel resumes, the risk of cross-border coronavirus disease 2019 (COVID-19) transmission persists.

The report, which was published in the Journal of the American Medical Association, is based on a pilot program that was launched at Qatar’s Hamad International Airport for two months earlier this spring.

The program allowed residents returning to Qatar via the airport to avoid mandatory quarantining so long as they were fully vaccinated and their second vaccination shot was at least 14 days before their arrival in the country. However, the passengers were also required to get a polymerase chain reaction (PCR) test upon arrival in order to confirm COVID-19 status.

Examining the resulting testing data, corresponding author Laith J. Abu-Raddad, PhD, of Weill Cornell Medicine-Qatar, said the results were not surprising.

“Indeed these findings were as expected building on the knowledge we have built about this infection and its epidemiology,” he told Contagion.

In line with existing study data, the findings showed that having previously been infected with the virus or having been vaccinated made an arriving passenger far less likely to be infected with COVID-19. Yet, vaccination did not totally eliminate risk.

More than a quarter-million people (261,849) arrived at the airport during the testing program, which ran from February 18 through April 26. The passengers were 75.1% male and had a median age of 33. Of those, 31,190 had been fully vaccinated. Of those, 10.092 could be matched by sex, age, nationality, and date of test with an unvaccinated person with no record of prior infection. By comparing the two groups, the investigators calculated a PCR positivity rate of 3.74% among the group that was unvaccinated and not previously infected, and a 0.82% positivity rate among the fully vaccinated passengers.

Similarly, 7694 of the 9180 passengers who arrived unvaccinated with a record of prior infection at least 90 days before the test could be matched with unvaccinated passengers with no known prior infection. Unvaccinated passengers with no previous infection had a COVID-19 positivity rate of 3.81%, compared with 1.01% among patients who had previously had COVID-19.

All told, Abu-Raddad and colleagues calculated a relative risk for a positive test at 0.22 for vaccinated passengers and 0.26 for passengers with a previous infection who had not been vaccinated.

Abu-Raddad said the data show the value of PCR testing, even in this phase of the pandemic. He said there are two key reasons policymakers should keep in mind for requiring PCR testing.

“[The] first is to prevent new variants from entering, as they could spark a new wave,” Abu-Raddad said. “Even in the presence of vaccination, the risk of some of the variants persists.”

He cited the United Kingdom’s recent struggles with COVID-19 variants as a prime example.

The other reason is to limit the number of potential new chains of transmission, a factor he said may be more or less urgent depending on the vaccination and transmission rates in a particular location.

In Qatar, the pilot program was not continued for practical reasons. Instead, passengers are required to get a test before arrival. Still, Abu-Raddad and colleagues wrote that the data make clear that despite the efficacy of the vaccines, breakthrough cases are a reality, and something that policymakers should consider as they re-examine travel restrictions and policies.

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