More COVID-19 testing is needed to gauge the prevalence of asymptomatic breakthrough infections, or those that occur among fully vaccinated people, according to Anthony Fauci, MD, the White House chief medical advisor.
Early CDC testing guidance called for people who had been in contact with an infected individual to be tested, as well as people who were symptomatic. That was prior to the emergence of more transmissible virus strains.
Now, with the cases of the delta variant surging in the U.S., there's a need for testing among asymptomatic people, Dr. Fauci, director of the National Institute of Allergy and Infectious Diseases, said during an Aug. 8 interview with NBC News' Meet the Press.
"Now, it's very clear you've got to go beyond that," Dr. Fauci said. "We know now, from experience here and in other countries, that you will have people who are asymptomatic, who get into contact with an asymptomatic person who is infected, and you'll know that there will be more infections that otherwise would have gone undetected."
While no vaccine is 100 percent effective and some breakthrough cases are expected, the current COVID-19 vaccines still largely protect against severe illness, Dr. Fauci said. Since breakthrough cases are largely mild or asymptomatic, experts are concerned that many breakthrough cases are missed, with infected people unknowingly transmitting the virus.
"The good news is that almost invariably [a breakthrough] will be an infection that is either without symptoms or minimally symptomatic, which means the vaccine still protects extremely well against severe disease leading to hospitalization and deaths."
However, because the delta variant is more transmissible and infected people carry more of the virus in their nasopharynx, vaccinated people can still transmit the virus, regardless of their symptom severity, he told NBC.
While the current COVID-19 surge is largely driven by people who are unvaccinated, allowing the virus to spread could eventually lead to the emergence of new variants that affect everyone, regardless of vaccination status.
"A virus will not mutate unless you allow it to replicate," Dr. Fauci said. "If you give the virus the chance to continue to change, you're leading to a vulnerability that we might get a worse variant, and then that will impact not only the unvaccinated, that will impact the vaccinated because the variant could evade the protection of the vaccine."
At the same time experts are calling for more testing to track asymptomatic cases, COVID-testing kit shortages are hitting regions with high caseloads, Kaiser Health News reports.