clock menu more-arrow no yes
We are finally starting to get some concrete answers about the Covid-19 vaccines and the delta variant.
Spencer Platt/Getty Images

What we actually know about the vaccines and the delta variant

Covid-19 vaccines still are effective in preventing hospitalization. But vaccinated people are more likely to get sick than before.

The Covid-19 pandemic has changed, and with it, so has the effectiveness of the vaccines.

The bottom line remains the same: The mRNA vaccines from Pfizer/BioNTech and Moderna that are most prevalent in the US are still quite effective in preventing any illness from the novel coronavirus, and extremely effective in preventing the kind of severe illness that leads to hospitalization and death. On Wednesday, the Centers for Disease Control and Prevention confirmed those basic facts with its most robust data yet.

But the statistics that were widely publicized when those vaccines were first approved in December — the ones that showed vaccines were 95 percent effective in preventing all illness and 100 percent effective in stopping hospitalization or death — are now thoroughly out of date. The risk that a vaccinated person would experience symptoms if they contract Covid-19 is higher than it was back then, even if it is still significantly lower than if the person were unvaccinated. The now-dominant delta variant is likely to blame.

So exactly how effective are the vaccines against this new, more dangerous iteration of the virus? And how long does immunity provided by the vaccines actually last?

We are finally starting to get some concrete answers to these questions. The vaccines did appear to lose some of their effectiveness in preventing any kind of illness as the delta variant became dominant, especially for people who are at highest risk from Covid-19. But the protection against severe illness held steady, according to the three CDC studies published this week.

The number of breakthrough infections is “increasing as the delta wave proceeds,” Eric Topol, director of the Scripps Research Translational Institute, told me. “But still, protection versus hospitalizations and deaths is very solid.”

With this new evidence of waning vaccine efficacy, the Biden administration announced plans this week to immediately make immunocompromised patients eligible for a booster shot and recommended that all vaccinated people get a third shot eight months after their second dose.

The new data and new guidance reflect this new chapter in the pandemic. Vaccinated people should still feel confident that they are protected against the worst outcomes from Covid-19. But the large number of unvaccinated Americans, and the delta variant’s potency, has contributed to surges in infections.

Some caution — continuing to wear masks and avoiding large indoor gatherings, for example — can help protect against the high level of spread currently in the US, experts said. Even with the powerful protection of vaccines, it’s possible to get sick after a coronavirus exposure.

We now know more than ever about Covid-19 vaccines and the delta variant

The Covid-19 vaccines were initially tested against the original version of the coronavirus, and they performed incredibly well. But delta has proven somewhat more capable of evading the vaccines and may cause more severe illness than its predecessors, based on early research out of the United Kingdom, one of the first places where delta took hold. The new CDC data is a big step forward because it brings our understanding of the vaccines closer to the present.

One of this week’s CDC studies tracked new cases and hospitalizations from early May to late July in New York. The study period covers the transition from the “alpha” variant to delta, which became dominant by the start of July, but only includes part of the recent surge in reported cases.

The vaccines grew less effective in preventing all illness as the delta variant took over, CDC researchers found. In May, vaccines had an estimated 90 percent effectiveness at preventing new cases. By mid-July, the estimated effectiveness had dropped to just under 80 percent. By that point, vaccinated people were more likely to get infected and actually feel sick. Breakthrough infections became more common.

But the vaccines have remained resilient against severe symptoms, with the estimated effectiveness against hospitalization holding steady around 95 percent from the start to the end of the study period.

“There was a reduction in vaccine effectiveness against SARS-CoV-2 infection, but not against hospitalization,” Celine Gounder, clinical assistant professor of medicine and infectious diseases at the NYU School of Medicine, told me. “The vaccines remain highly protective against hospitalization in all age groups.”

It’s important to remember that “severe” illness is a clinical term that might not align with the common parlance. A vaccinated person who becomes ill with Covid-19 might still feel very sick.

“Severe disease isn’t that you feel sick like a dog and are laid up in bed,” Gounder said. “Severe disease means your lungs are failing, your oxygen levels are dropping, and you need to be in the hospital.”

In other words, the vaccines have gotten less effective at stopping Covid-19 in its tracks but are still extremely good at protecting people from the kind of severe cases that need hospital beds or ventilators. Many patients with breakthrough infections can recover at home.

How the vaccines affect long Covid remains an open question. Preliminary evidence seems to suggest that they help alleviate (but not always eliminate) those long-term symptoms.

Why are vaccines somewhat less effective against infection than before?

The CDC researchers were careful to say the reasons for diminishing vaccine effectiveness are uncertain, but there are some things they’re confident about. The delta variant causes a substantially higher viral load than its predecessors — there is more of the virus when a person gets infected — and the sheer amount of the virus a person contracts may play a role. At the same time, people are now taking fewer precautions against Covid-19 than they were last fall and winter, the researchers said, making it more likely that they’ll be exposed to a high viral load.

Or to think of it another way: The rapid spread of delta in the unvaccinated population means vaccinated people are getting exposed to the virus more often, and being exposed to more of it than they were before.

Data from other countries shows a wide range of vaccine effectiveness against infection with the delta variant, but studies have generally found the protection is less robust than it was against the alpha variant. Still, in all cases, the vaccines available in the US continue to impress in their ability to prevent the worst outcomes.

A second CDC study examined national data to determine whether the vaccines are becoming less effective at stopping severe illness over time. Like the New York study, it found that vaccines are extremely effective — about 90 percent at preventing hospitalization due to Covid-19.

A mobile vaccination site in Orlando. With this new evidence of waning vaccine efficacy, the Biden administration announced plans this week to immediately make immunocompromised patients eligible for a booster shot.
SOPA Images/LightRocket via Getty Images

Reassuringly, there was not a meaningful difference in the vaccine’s ability to stop hospitalization as time wore on. The researchers estimated the vaccine’s effectiveness against hospitalization in two time periods: two to 12 weeks after patients received a second dose, and 13 to 24 weeks after that second dose.

They did not find a meaningful decline almost six months after patients received a second dose of the vaccine — which is very good news.

“There was no reduction in vaccine effectiveness over time,” Gounder said, “which demonstrates that protection against hospitalization did not drop over time or after the emergence of the delta variant.”

The best protection for the most vulnerable is everybody getting vaccinated

While the vaccines have generally held up well against the delta variant, some of the people most vulnerable to Covid-19 do not receive the same level of protection.

For people with compromised immune systems, the CDC researchers found that the vaccines were less effective at preventing hospitalization. That finding supported the Biden administration’s plan to make those people immediately eligible for a third shot, experts said.

For immunocompromised individuals, it’s also possible that immunity may wane more over time — but that didn’t happen, according to this research. Their level of protection appeared to be constant during the six-month period covered by the study.

The third CDC study evaluated vaccine effectiveness for nursing home residents, a population particularly vulnerable to Covid-19 and one of the first groups to get vaccinated at the beginning of this year. That study did find declining effectiveness over time against any illness for those Americans, from 75 percent pre-delta to about 50 percent post-delta.

That drop-off may partly reflect the nature of this population. Older people’s immune systems are not as strong as younger people’s; older adults already experienced a lower baseline vaccine effectiveness rate than the general population before delta took over (75 percent versus 90 percent). The decline also probably reflects the basic fact that the delta variant is better at evading the vaccines than the alpha variant was.

“It makes sense to give an extra dose of vaccine to vaccinated nursing home residents,” Gounder said. “But what will have an even bigger impact on protecting those nursing home residents is to vaccinate their caregivers.”

As of late July, about 60 percent of nursing home workers had been vaccinated, substantially lower than the 80 percent rate among residents. The Biden administration announced on Wednesday that it would require nursing homes to mandate all their workers be vaccinated if the facilities want to receive federal health care funding.

Over the long run, so long as a substantial portion of the US population remains unvaccinated, there will be risks to everybody. Currently, 72 percent of the 18-and-over population and 60 percent of the entire US population are vaccinated, according to the New York Times tracker. That leaves millions of people without protection against the virus. Some of them are children not yet eligible for the vaccine, but millions of people who are currently eligible and could receive the vaccine for free still haven’t done so.

“Your risk depends on your vaccination status and what’s happening in your community,” Gounder told me. “Vaccines aren’t an immunity on/off switch for individuals. Vaccines work additively and synergistically across populations.”

Gounder deployed some hypothetical math to explain how risk works under different vaccination scenarios. If a country has a baseline of 1 million “units” of risk for each person, because the virus is very widespread, a 95 percent effective vaccine would reduce that risk to 50,000.

But if the baseline risk is 100, because the country contains the virus through vaccinations and mitigation measures, the vaccinated person faces just 5 units of risk. That huge difference in risk depends on how rampantly the virus is spreading in the overall population.

“This isn’t about individualism, individual rights, individual responsibility, and individual protection,” Gounder said. “This is about community immunity.”

Recode

How Walgreens’ sloppy Covid-19 test registration system exposed patient data

Explainers

Why Biden’s approval numbers have sagged, explained by an expert

Explainers

The redistricting wars

View all stories in Explainers