Should vaccinated travellers be tested for COVID-19 to prevent asymptomatic spread?

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Airlines need to be able to prevent the spread of COVID-19 to restart mass foreign travel. Some organisations are considering using vaccine passports.

Although the new COVID-19 vaccines are successful at preventing the COVID-19 syndrome, it is still unknown whether the vaccines prevent people from getting infected and passing on the SARS-CoV-2 virus.

Additionally, many countries, including the U.S., require fully vaccinated air travellers from abroad to have a negative SARS-CoV-2 viral test result or documentation of recovery from COVID-19 before they board a flight to the United States (1).

Testing passports based on sensitive testing are a critical component of restarting air travel.

Vaccinated people can be asymptomatic carriers

Preclinical studies of COVID-19 vaccines demonstrated persistent virus in nasal swabs, even after the subject had been vaccinated.

During the trial of Moderna’s vaccine, researchers saw a significant drop – but not elimination – in the number of asymptomatic infections among people who received the first shot of the two-dose vaccine, compared with those who received a placebo (2).

During a trial of the AstraZeneca vaccine researchers swabbed participants every week, and estimated a 49.3% reduction in asymptomatic infections among a subset of vaccinated participants, compared with the unvaccinated group (3). This data suggests people can have asymptomatic infections even after being vaccinated.

Viral load is reduced in vaccinated individuals

Researchers have noted a significant drop in viral load in people who were infected with SARS-CoV-2 in two to four weeks after receiving their first dose of the Pfizer vaccine, compared with those infected in the first two weeks after the injection (4).

The AstraZeneca trial also observed a larger reduction in viral load in a group of vaccinated participants than in the unvaccinated group (5).

Low limits of detection critical to detection of samples with low viral loads

If the viral load of a sample is lower than the LoD (limits of detection) or cut-off value of a real-time RT-PCR assay, a false negative result will be obtained.

Each tenfold increase in the LoD of a COVID-19 viral diagnostic test is expected to increase the false negative rate by 13% (6). Therefore, an assay with low LOD (high sensitivity) is critical for accurate detection of samples with low viral load (7).

Vaccinations reduce, but do not eliminate the chance of viral spread

CDC states that while the risks of SARS-CoV-2 infection in fully vaccinated individuals and transmission to others is reduced, it cannot be eliminated in the setting of continued widespread community transmission of the virus.

Vaccinated people could potentially still become infected and spread the virus to others (8). 

Testing vaccinated individuals for asymptomatic infections

Since vaccinated individuals have a reduced viral load, to check to see if they are asymptomatic carriers a highly sensitive test with a low LOD must be used to prevent the reporting of false negatives.

The FDA developed an experiment for precise comparison of the performance of the nucleic acid-based SARS-CoV-2 assays, which have received EUA authorisation, and published a comparative performance analysis.

This assessment used the FDA SARS-CoV-2 reference panel, which allowed a consistent determination of the relative sensitivity of these tests.

See how the assays rank in comparison to each other (9). 


  1. CDC. Interim Public Health Recommendations for Fully Vaccinated People.
  2. Food and Drug Administration. Moderna COVID-19 Vaccine. Vaccines and Related Biological Products Advisory Committee December 17, 2020 Meeting Briefing Document Addendum – Sponsor.
  3. AstraZeneca. COVID-19 Vaccine AstraZeneca confirms 100% protection against severe disease, hospitalisation and death in the primary analysis of Phase III trials.
  4. Levine-Tiefenbrun, M., Yelin, I., Katz, R. et al. Initial report of decreased SARS-CoV-2 viral load after inoculation with the BNT162b2 vaccine. Nat Med (2021).
  5. Oxford says COVID-19 vaccine with AstraZeneca works against UK variant.
  6. Arnaout R. et al. SARS-CoV2 testing: the limit of detection matters. Preprint at bioRxiv.
  7. FDA. A Closer Look at Coronavirus Disease 2019 (COVID-19) Diagnostic Testing.
  8. CDC. Science Brief: Background Rationale and Evidence for Public Health Recommendations for Fully Vaccinated People
  9. FDA Reference Panel Comparative Data. April 6, 2021.

For emergency use authorisation only.

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