Vaccines Not as Effective Against Delta Variant, say CDC Data


CDC data cut to the heart of just how much protection COVID-19 vaccines offer infection preventionists (IPs) and other health care professionals on the frontlines from the delta variant.

A trio of studies—2 by the Centers for Disease Control and Prevention (CDC) released yesterday, and 1 by Oxford University investigators released last week—seem to indicate that the effectiveness of COVID-19 vaccines wanes in the face of the delta variant. Investigators in the 3 studies agree that their findings present an argument that as many people as possible should be vaccinated against COVID-19.

One of the CDC’s studies cuts to the heart of just how much protection COVID-19 vaccines offer infection preventionists (IPs) and other health care professionals on the frontlines. Vaccine effectiveness in protecting frontline workers decreased nearly 30 percentage points since the delta variant became the dominant strain.

The study, published in the CDC’s Mortality and Morbidity Weekly Report (MMWR), also says that despite evidence of waning, the vaccines are still 80% effective in protecting frontline health care workers.

The CDC investigators, who belong to the CDC COVID-19 Response Team, looked at data from reverse transcription–polymerase chain reaction (RT-PCR) tests for 4217 health care workers, first responders and other frontline workers in 8 locations across 6 states. The delta variant was considered the dominant strain when it accounted for over 50% of cases around those locations.

Of the participants, as of August 14, 2021, 3483 (83%) were vaccinated: 2278 (65%) with Pfizer/BioNTech, 1138 (33%) with Moderna, and 67 (2%) with Janssen.

Vaccine effectiveness declined from 91% before the delta variant became the dominant strain to 66% since it became dominant. The study states that this “trend should be interpreted with caution because VE [vaccine effectiveness] might also be declining as time since vaccination increases and because of poor precision in estimates due to limited number of weeks of observation and few infections among participants.”

And, again, the authors reiterate that although “these interim findings suggest a moderate reduction in the effectiveness of COVID-19 vaccines in preventing infection, the sustained two thirds reduction in infection risk underscores the continued importance and benefits of COVID-19 vaccination.”

These findings come at a time of rising breakthrough infections, according to another CDC study in MMWR. That study, also released yesterday, shows that about 25% of COVID-19 infections in Los Angeles country from May through July 25 occurred in fully vaccinated people.

When the delta variant was dominant “on July 25, infection and hospitalization rates among unvaccinated persons were 4.9 and 29.2 times, respectively, those in fully vaccinated persons.” These data were crunched by the Acute Communicable Disease Control Program, Los Angeles County Department of Public Health. They looked at data from 43,127 people infected by COVID-19 in Los Angeles Country from May 1, 2021, to July 25, 2021. Of that population, 10,895 (25.3%) were fully vaccinated and 1431 (3.3%) were partially vaccinated. Meanwhile, 30,801 (71.4%) were unvaccinated.

“In July, when the delta variant was predominant, cycle threshold values were similar for unvaccinated, partially vaccinated, and vaccinated persons,” the study states.

The authors conclude that “efforts to increase COVID-19 vaccination coverage, in coordination with other prevention strategies, are critical to preventing COVID-19–related hospitalizations and deaths.”

It’s the “other prevention strategies” that might be revisited to try and beat back the delta variant, strategies already being undertaken in some states. Those include mask mandates, social distancing, and at least partial lockdowns.

Kevin Kavanagh, MD, a member of Infection Control Today®s (ICT®)’s Editorial Advisory Board, made the same argument on July 27 in ICT® in a Q&A headlined “Everybody Needs to be Vaccinated and Wear a Mask.” Kavanagh said that “I am convinced this virus is about one or two iterations away from completely avoiding the vaccine. And remember, we have the lambda variant and the kappa variant which are sitting out there in the wings, waiting for immunity to drop and possibly cause another wave.”

A study in the United Kingdom also calls into question just how long vaccines can stay effective. In a preprint study, investigators with Oxford University and the UK’s Office for National Statistics, state that the Pfizer/BioNTech and Oxford/AstraZeneca vaccines are effective against the delta variant, but that their protection decreases over time. The investigators looked at data from 2,580,021 PCR tests taken by 384,543 UK adults between December 1, 2020 to May 16, 2021—when the original wild type variant was dominant—and 811,624 test results from 358,983 people between May 17, 2021 and August 1, 2021, when the delta variant was dominant. The Pfizer/BioNTech vaccine was 92% effective in keeping people from getting a high viral load, However, the effectiveness waned to 85% after 60 days and 78% after 90 days. The Oxford/AstraZeneca vaccine was 69% effective against a high viral load 14 days after the second dose. That dropped to 61% after 90 days.

As is the case with the CDC study, investigators in the UK study say that their findings bolster the argument for vaccinating as many people as possible. Sarah Walker, PhD, the lead investigator stated in an Oxford University press release that “the fact that they can have high levels of virus suggests that people who aren’t yet vaccinated may not be as protected from the Delta variant as we hoped. This means it is essential for as many people as possible to get vaccinated—both in the UK and worldwide.”

Not all experts feel that the situation is as troublesome as some interpretations of the studies' findings indicate. One of those experts is Muge Cevik, PhD, an infectious disease expert at the University of St. Andrews in Scotland and a scientific adviser to the British government. Cevik told the New York Times that while Britain as seen breakthrough infections, they’ve happened mostly to people exposed to large amounts of the virus—such as health care workers.

“Vaccine efficacy measures the relative reduction in infection/disease for the vaccinated vs unvaccinated arm,” Cevik tweeted. “For instance, a vaccine that eliminates all risk would have an efficacy of 100%. Efficacy of 50% means you have a 50% reduced risk compared to an unvaxxed person.”

Cevik says that vaccines are designed to protect against symptomatic COVID-19, and that’s often the measure of vaccine effectiveness. However, some studies also report VE against infection and hospitalization.

“The evidence so far suggests that VE against hospitalisations is highly preserved against Delta,” Cevik tweeted. “….[H]igh protection against hospitalisation (>80%) is maintained after the 2nd dose.”

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