Why breakthrough infections matter, and what we can do about them

By Lindsay Williams, PhD

Since COVID-19 vaccines became available in December 2020, medical experts have remained adamant that inoculations are the best protection against the virus. This remained the case even as the hyper-aggressive delta variant became the dominant strain of SARS-CoV-2 in our country.

The statistics have largely supported the vaccine’s efficacy, even as the U.S. vaccination rate hit a midsummer slowdown. But the COVID-19 vaccine (like any other) is not 100% effective.

Look no further than last month’s outbreak in Provincetown, Massachusetts. In that instance, the Centers for Disease Control and Prevention (CDC) identified 469 positive COVID-19 cases among state residents who had visited the town during a two-week stretch.

The kicker? A whopping 74% of those cases were in fully vaccinated individuals.

While an extreme example, the Provincetown outbreak illustrates just how wary we should be of breakthrough cases— coronavirus infections that occur in inoculated people. More and more of these phenomena have emerged thanks to the high transmissibility of the delta variant, leading the CDC to say “the war [against the coronavirus] has changed.”

Knowing how breakthrough cases occur, what threats they pose and how we can prevent them is essential to continuing our global pandemic recovery.

How do breakthrough infections happen?

Even highly successful COVID-19 vaccine brands will fail to prevent infection from time to time, a risk that becomes gradually more likely after the vaccine is administered. For instance, the CEO of Pfizer, said its vaccine drops from 96.2% effectiveness to about 84% after four to six months.

Even with such failure rates considered, vaccinated people have made up a fraction of coronavirus infections, related hospitalizations and deaths since last December. Vaccinated people are, overall, three times less likely to be infected and 10 times less likely to suffer severe disease or death as a result, according to the CDC. (The agency stopped tallying breakthrough cases in May.)

Health experts have pinpointed what makes the delta variant particularly contagious, even among the inoculated: its high viral load. As my colleague, Chief Medical Officer Christopher Reeves, MD, noted last month, those who contract the delta variant average about 1,000 times more copies of the virus in their respiratory tracts than those infected with the initial strain.

That means you need much less of the delta variant in your system to produce a positive COVID-19 test result. And once you do contract the delta variant, you pose significant risk to those around you — whether you’re vaccinated or not.

What risks do breakthrough infections pose?

While vaccination significantly lowers the risk of contracting any coronavirus variant, breakthrough cases involving the delta variant can be just as dangerous and contagious as cases occurring in unvaccinated individuals.

A CDC study found that vaccinated and unvaccinated people who are infected with the delta variant are equally likely to spread it to someone else. Experts drew this conclusion based on similar viral loads present in breakthrough infections and other cases.

Vaccinated individuals who contracted earlier, tamer coronavirus variants were not able to transmit the virus, according to CDC Director Dr. Rochelle Walensky. Now, both symptomatic and asymptomatic carriers can pass the delta variant along, although symptoms tend to be milder or absent in vaccinated people.

Walensky said this discovery informed the CDC’s decision to update its masking recommendations last month, recommending that even vaccinated people mask up in high-transmission areas. Some localities have reactivated preventive measures to address the nationwide case uptick, with New York City now requiring proof of vaccination for those partaking in indoor dining, fitness and entertainment.

More restrictions are bound to emerge as breakthrough cases interrupt operations in schools, travel situations and other areas of day-to-day life. For example, seven breakthrough cases were reported among Stanford University students in late July, as well as four passengers on a Royal Caribbean cruise ship the following week.

However, as with other coronavirus variants (and infectious diseases in general), those who are elderly or immunocompromised are at greatest risk of breakthrough infections. Out of the 7,525 reported U.S. hospitalizations or deaths in vaccinated individuals as of Aug. 2, people aged 65 or older made up 74%.

Even with the relative rarity of breakthrough infections, as a clinician, I can’t ponder those exceptions without thinking about the harm, disability or death that can result. We’ve become largely desensitized to the alarming overall COVID death toll these last 17 months, so 1,500-plus reported deaths from breakthrough infections may seem fractional. But each of those represents a mother, father, daughter or son who will be mourned by those he or she leaves behind.

All fatal breakthrough cases — no matter how rare they may be — are devastating and needless … and ultimately preventable.

How can we prevent breakthrough infections?

The CDC now recommends even fully vaccinated people be tested after being exposed to someone who is confirmed positive. But aside from known exposures, testing is a vital asset for staying on top of the spread of the virus.

While some localities have re-upped their health mitigation mandates, many cities and counties haven’t gone beyond simple masking or social distancing recommendations, making virus spread that much easier in dense public areas like restaurants, theaters and stores. Getting a reliable PCR test – which Worksite Labs offers through our convenient drive-through sites – is the best way people can protect others after any situation involving a large number of people.

Testing is also a common-sense precursor to visiting family or friends who are elderly, immunocompromised or otherwise at high risk from COVID-19, even if they’re vaccinated. Those who work in-person or use public transportation alongside dozens of others can best serve their at-risk loved ones by making sure they’re not carrying anything potentially deadly.

Anyone who can’t get a shot in the arm due to age or other personal or health restrictions can turn to testing as a reliable means of protection. That also applies to those who are vaccine-hesitant and wish to keep themselves and others safe from the virus.

Finally, it’s important during this uptick in COVID-19 infections and breakthrough cases to remember that there’s no such thing as “over-protection.” Even if someone isn’t required to get tested, mask up or stay 6 feet from fellow bus passengers, nothing is stopping them from taking those measures anyway. PCR testing is the most fail-safe way to keep from getting infected, but everyone should implement whatever level of protection feels right — their peace of mind is worth it.

The rise of breakthrough infections is just the latest curveball in COVID-19’s plan of attack. With dedicated testing, we can stay one step ahead of whatever the virus’ next play might be.

Lindsay Williams, PhD, is chief clinical officer of Worksite Labs.

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