‘Breakthrough infections’ get tracked

Post-vaccine illness cases are rare, but CDC is monitoring situation

As tens of millions of people in the United States reach the coronavirus vaccination finish line, a small fraction have had “breakthrough infections,” testing positive for the virus after being inoculated and in rare cases requiring hospitalization, according to data from state health departments.

The precise number of these breakthrough cases is unknown, but figures released by states suggest it is at least several thousand. The Centers for Disease Control and Prevention, which has had a team monitoring breakthrough infections since February, has partial data but has not made it public.

These cases represent a tiny percentage of the 66 million people fully inoculated, and experts say they are neither unexpected nor occurring at an alarming rate. Indeed, the rarity of the breakthrough illnesses in the context of the vast scale of inoculations reinforces the encouraging message from public health experts: These vaccines are highly effective, and their rollout has dramatically driven down the rates of sickness and death among the most vulnerable populations first targeted for inoculations.

“There’s nothing there yet that’s a red flag,” Anthony Fauci, President Joe Biden’s chief medical adviser for the pandemic, said at a White House news briefing Friday when asked about the breakthrough cases. “We’re obviously going to keep an eye on that very, very carefully.”

The administration, state health officials and front-line health care workers face challenges in trying to get a clear picture of these outlier cases:

The data is incomplete. Some states have not reported their breakthrough infection numbers to the CDC. Some post-vaccination deaths are still under investigation and may not be caused directly by COVID-19, the disease caused by the coronavirus.

Investigators are also hampered by the scattered and often chaotic nature of the pandemic response in the United States. Genomic sequencing is critical to knowing which strain of the virus causes an infection, but positive test samples are often discarded before investigators can retrieve them. Vaccinated people may also be less motivated to get tested if they come down with a case of the sniffles — they might assume it’s just a cold rather than COVID-19.

There is no singular explanation for why the virus in rare cases is not neutralized by the vaccine-induced immune response. Infectious-disease experts say the human immune system is complicated, and some people may simply have a weak immune response to the vaccine. In this scenario, it’s not the vaccine that’s the wild card, it’s the patient.

Fauci offered that explanation Friday during the White House briefing. The several deaths reported so far among people already fully vaccinated were among elderly individuals who may have had underlying health conditions and may not have mounted a strong immune response when vaccinated, he said.

“I don’t think there needs to be any concern about any shift or change in the efficacy of the vaccine,” Fauci said.

A less-likely scenario is that mutated variants of the virus are finding ways to evade immunity induced by vaccines. Investigators generally lack comprehensive data matching specific breakthrough infections with specific strains of the virus.

These cases represent a tiny percentage of the 66 million people fully inoculated, and experts say they are neither unexpected nor occurring at an alarming rate.