As the SARS-CoV2 the virus behind the COVID-19 infections is evolving rapidly and producing new variants, continuing testing of immunised individuals will be important to help mitigate future outbreaks, say researchers.

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"The idea that we could be entirely done with testing in the post-vaccine world is probably not a good one right now; for example, even fully vaccinated people who develop respiratory symptoms should consider getting tested for COVID-19," said lead researcher Robert B. Darnell, Professor at the Rockefeller University in New York, US. The study, published in the New England Journal of Medicine, reports results from ongoing monitoring at the varsity where two fully vaccinated individuals tested positive for the coronavirus.

Both had received two doses of either the Moderna or the Pfizer vaccine, with the second dose occurring more than two weeks before the positive test. One person was initially asymptomatic and then developed typical COVID-19 symptoms, the other developed symptoms prior to testing. Both individuals recovered at home, an outcome consistent with evidence suggesting vaccination is effective in preventing severe disease. Genome sequencing revealed multiple mutations in both viral samples, including the E484K variant in one individual, first identified in South Africa and Brazil, and the S477N variant in the other individual, which has been spreading in New York since November.

The researchers were able to discern a quantifiable amount of virus in saliva samples from routine testing and sequence the viral RNA. The observations suggest a small but ongoing risk among vaccinated individuals and the possibility that they may continue to spread the virus. "Conversely, exposure to individuals with known infection, even if fully vaccinated, should be taken seriously and again individuals should consider getting tested". "Given the scope of the pandemic, there's a huge amount of virus in the world right now, meaning a huge opportunity for mutations to develop and spread", he adds. "That is going to be a challenge for the developers of vaccines over the next months and years".