COVID-19 Vaccines Prevent Delta Transmission, Yet Another Study Shows

Over the course of the pandemic, our understanding of SARS-COV-2 has changed almost weekly as new evidence has continued to roll in. The emergence of several viral variants has only accelerated this information overload, making it difficult for anyone to keep up with what the science says. With that in mind, I want to briefly review what we know up to this point about the efficacy of COVID-19 vaccines against the dreaded delta variant.

It's my contention that these shots continue to prevent severe disease and greatly reduce transmission of the virus. Wearing masks everywhere we go and treating each other like walking petri dishes to avoid are behaviors driven by risk-aversion, not a careful reading of the evidence. Ironically, both anti-vaccine groups and many in the mainstream science community will object to this assessment of vaccine efficacy, so we'll deal with their concern about transmission from breakthrough cases as well. 

The latest evidence

The latest evidence we have that vaccination keeps delta in check comes from a just-published study conducted in the UK. The paper is worth reading in its entirety, but here's the gist. The researchers investigated breakthrough infections in thousands of partially and fully vaccinated individuals. There were two case groups, one for participants who had received one dose and developed a subsequent infection 14 days later and a second group for fully vaccinated persons who also became infected seven days later. These two case groups were matched with two vaccinated control groups who did not experience breakthrough infections. Results:

Between Dec 8, 2020, and July 4, 2021, 1,240,009 COVID Symptom Study app users reported a first vaccine dose, of whom 6,030 (0·5%) subsequently tested positive for SARS-CoV-2 (cases 1), and 971,504 reported a second dose, of whom 2,370 (0·2%) subsequently tested positive for SARS-CoV-2 (cases 2).

So, breakthrough infections were incredibly rare. But what about disease severity in those rare cases? Participants from both vaccinated case groups were sub-selected and compared to two unvaccinated control groups based on “the date of the positive COVID-19 test, health-care worker status, sex, body-mass index (BMI), and age.” The researchers found that:

Almost all individual symptoms of COVID-19 were less common in vaccinated versus unvaccinated participants, and more people in the vaccinated than in the unvaccinated groups were completely asymptomatic … We also found that COVID-19 was less severe (both in terms of the number of symptoms in the first week of infection and the need for hospitalization) in participants after their first or second vaccine doses compared with unvaccinated participants.

What do other studies show?

Previous research has reached the same results. Another recent study conducted in the UK found that the existing vaccines prevented the majority of infections, including those caused by delta. Five studies published between May and July agreed. Even in a transmission-friendly setting like an infectious disease hospital, breakthrough cases in immunized healthcare providers were rare and relatively mild, with only one requiring oxygen supplementation for three days.

These results are likely explained by the fact that immunization dramatically reduces viral load in vaccinated individuals or helps them clear infectious viral particles quicker, both of which reduce disease severity and the chances of infecting other people.

If you don't believe me, consider this recent report on breakthrough cases from the CDC. “Although these interim findings suggest a moderate reduction in the effectiveness of COVID-19 vaccines in preventing infection,” the agency wrote, “the sustained two-thirds reduction in infection risk underscores the continued importance and benefits of COVID-19 vaccination.” State-level data on US breakthrough cases show the same outcome: these infections are incredibly rare. 

I know, I know. But Israel!

Israel has become the poster child for vaccine failure, though, contra popular media reports, their experience has been overblown. Stratifying breakthrough data by age paints a less apocalyptic picture of their immunization drive. The country has also recently rolled out booster shots, which have cut infection risk more than 10-fold, Science reported on September 1. Whether or not booster doses provide long-term protection is still unknown, but we have to work from the data we have, not the data we're afraid we might have one day.

It's also not clear that other countries will need boosters. Some experts have pointed out "that while protection against any infection does start to fade, the vaccines are still extremely good at doing what they were designed to do: Keep people out of the hospital and the morgue."  

Masks and social distancing necessary?

Despite all this evidence of vaccine efficacy, some experts still argue that the higher rate of asymptomatic cases among the vaccinated “might support caution around relaxing physical distancing and other personal protective measures in the post-vaccination era,” as the Lancet authors put it.

But if we accept the results of the aforementioned studies, there is no justification for continued mandatory masking and social distancing for the majority of people, who are now protected thanks to vaccination and infection-induced immunity. Indeed, a study published on September 2 found that more than 80 percent of Americans likely have some level of protection against the virus, thanks to a combination of vaccines and infection.

The counterargument, in so many words, is that we need to fall back on non-pharmaceutical interventions (NPIs) that we used as stopgap measures throughout 2020, with very limited success. The evidence that masking, for example, significantly reduces viral transmission is inconsistent and very messy, as we've discussed in great detail recently. According to hematologist and oncologist Vinay Prasad, an associate professor of epidemiology and biostatistics at UC San Francisco, the limited efficacy of masking is especially clear when we consider its impacts on young children.

This isn't to say that NPIs do nothing. The point is that we can't wantonly halt normal existence, say, by locking down an entire continent. That's not sensible public health policy; it's paranoia. Their broad endorsement of NPIs notwithstanding, the new Lancet paper authors struck a sensible balance elsewhere in their study:

Our results ... highlight the need to balance personal protective measures in those at risk of post-vaccination infection with the adverse effects from ongoing social restrictions. Strategies, such as timely prioritization of booster vaccinations and optimized infection control measures, could be considered for at-risk groups. Research is also needed on how to enhance the immune response to vaccination in those at higher risk of post-vaccination infection.

Put another way, vaccination remains our best weapon against COVID-19, perhaps paired with targeted use of NPIs to protect people who face an elevated risk. Insisting that everyone continue following these measures contradicts the very clear evidence of vaccine efficacy and gives skeptics a perfect excuse to avoid vaccines, now and in the future.