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Vaccines Q & A

Q&A: Delta Variant, Masks and Breakthrough Infections

Confusion abounds in the United States as citizens worry about breakthrough infections among fully vaccinated individuals and struggle to understand new recommendations about masking from the U.S. Centers for Disease Control and Prevention.

William Moss, executive director of the Johns Hopkins International Vaccine Access Center, addresses a variety of the most pressing concerns as the more contagious and severe Delta variant drives surges in cases, hospitalizations and deaths, predominantly among unvaccinated people.

William Moss, Executive Director, International Vaccine Access Center, JHU
July 30, 2021

Why are fully vaccinated people getting COVID-19?

When fully vaccinated people contract COVID-19 we call that a breakthrough infection. People have to understand that while breakthrough infections are rare, they are also to be expected with any vaccine.

Vaccines rarely, if ever, have what we call “sterilizing immunity” where they completely prevent infection.

Vaccines act differently than masks, which serve as barriers that can actually prevent the virus from getting into our bodies to infect our cells.

Vaccines don’t do that. The virus can still enter our bodies and infect our cells after we are vaccinated. The power of vaccines come after that point. That’s when our immune systems – which have been primed by the vaccines – kick in to fight off the infection. Sometimes the immune system can rapidly clear the infection before there is much viral replication. But other times there is some viral replication.

“The current vaccines provide very high protection against the Delta variant.”

Some people will have mild symptoms while many remain asymptomatic. The vast majority, however, will avoid suffering from severe disease that leads to hospitalization and possible death. Our vaccines are protective against severe disease.

For more on breakthrough infections, see Breakthrough Infections Are Rare But Expected

Are the current vaccines effective against the Delta variant?

Yes. Very effective, in fact. Two shots of the Pfizer vaccine are 88% effective against the Delta variant in preventing symptomatic disease. That compares with 94% effectiveness against the Alpha variant. It’s a little lower, but it is still very high protectiveness for any vaccine. It’s far higher than the effective rates of the vaccines developed against seasonal flu.

We would need to worry if the effectiveness were something like 30%. That’s when we would definitely need booster shots or even a new vaccine specifically targeting the Delta variant. But we’re not in that situation. The current vaccines provide very high protection against the Delta variant.

For more on boosters, see Will Vaccinated People Need Boosters?

When will the United States achieve herd immunity?

I’m often hesitant to raise the issue of herd immunity because it’s a much more nuanced concept than we generally talk about. It’s related to the basic reproductive number of the virus, which is on average how many other individuals are infected from a single case. With the original variant of SARS-CoV-2, the reproductive number was about 2.5 – so each single case would infect 2.5 other people. To take the simplest formula, the herd immunity threshold for such a reproductive number would be about 60%. That’s the number we were talking about at the beginning of the pandemic.

“Breakthrough infections are rare, they are also expected with any vaccine.”

But the Delta variant has doubled that reproductive number to about 5 or 6. That gets the herd immunity threshold up to 80 or 85%. We’re nowhere near that in the United States. Just 50% of the U.S. population is fully vaccinated.

Now if all adults had achieved the vaccination coverage that adults over age 65 have achieved – which is 80% -- we would be far closer to that magical herd immunity threshold. The only way to get close to that is for the United States to get back to vaccinating the amount of people every day that it was at the start of the rollout – about 3 million. Today we’re vaccinating on average just 300,000 people a day.

Anyone who doubts the vaccines should remember where we were at the start of the vaccination rollout in January: about 250,000 new cases and 3,500 deaths every day. That was before the Delta variant. It’s hard to imagine that right now thanks to the level of vaccination coverage we’ve achieved. But the new cases and hospitalizations are being driven primarily among the unvaccinated parts of the nation.

For more on herd immunity, see No Magic Number for Herd Immunity.

Why did the CDC change its masking recommendations for fully vaccinated people?

The Delta variant was not as dominant when the CDC lifted its masking recommendations for fully vaccinated people.

Now that the Delta variant is the predominant version of the virus – and, most importantly, the fact that case numbers, hospitalizations, and deaths are increasing – the CDC changed those recommendations knowing, as we discussed above, that fully vaccinated individuals can carry and transmit the virus.

What more can the government do to encourage people to get vaccinated?

The U.S. Food and Drug Administration has not given any vaccine full approval. Currently they are all being administered under an EUA, or emergency use authorization. Many people still consider the vaccines to be experimental because they’re under that EUA.

A full FDA approval might sway some people to accept vaccinations. But that full approval would have the biggest impact on providing the support employers, school systems, and others will need to require vaccinations.

“New cases and hospitalizations are being driven primarily in the nation’s unvaccinated regions.”

Do I think the vaccines need full FDA approval to prove that they’re safe and effective? No. An EUA does not mean the vaccines are less safe. The same safety protocols were followed as with all other vaccine trials. The fact that we were able to test them on so many people so quickly, in large part because they were tested during a pandemic when case numbers were high – allowing us to quickly see the differences in case numbers between those who received the vaccine and those who received the placebo, provided enough evidence that they are safe and efficacious.

More than 1 billion people around the world have been vaccinated and 164 million people have been vaccinated in the United States. The Centers for Disease Control and Prevention has been tracking adverse events through its V-safe program and the FDA’s tracking system is the Vaccine Adverse Event Reporting System.

The United States initiated a pause in the Johnson & Johnson vaccine after identifying a rare blood clotting disorder in just six people who had received the vaccine. The system is working like it would with any vaccine.

For more on vaccine development, see Vaccine Research & Development
For more on vaccine hesitancy, see People Hesitant to Receive Vaccines Need Empathy

William Moss, Executive Director, International Vaccine Access Center, JHU

William Moss, MD, MPH, is Executive Director of the International Vaccine Access Center, Johns Hopkins Bloomberg School of Public Health.