What Is a Breakthrough Infection? What You Need to Know About Catching COVID-19 After Vaccination

Vaccines do not ward off every infection, but they greatly reduce the risk.

If you have been fully vaccinated against COVID-19, you may have thought that you no longer had to worry about contracting the coronavirus. But along with the increasing number of new COVID-19 cases worldwide and growing concerns about highly transmissible strains like the delta variant, there are reports of fully vaccinated people testing positive for COVID-19.

Members of the New York Yankees, US Olympic gymnast Kara Eaker and British Secretary of Health Sajid Javid are among those diagnosed with what has been called a “breakthrough infection.”

As scary as the term sounds, the bottom line is that existing COVID-19 vaccines are still very good at preventing symptomatic infections, and breakthrough infections are very rare. But how common and how dangerous are they? Here’s a guide to what you need to know.

What is ‘breakthrough infection’?

No vaccine is 100% effective. Dr. Jonas Salk’s polio vaccine was 80-90% effective in preventing paralysis disease. Even for the gold standard measles vaccine, efficacy was 94% in a highly vaccinated population during major outbreaks.

By comparison, clinical trials showed that Pfizer and Moderna’s mRNA vaccines were 94%-95% effective in preventing symptomatic COVID-19 — much more protective than initially hoped.

A quick reminder: a vaccine effectiveness of 95% does not mean that the injection will protect 95% of people, while the other 5% will contract the virus. Vaccine efficacy is a measure of relative risk – you need to compare a group of vaccinated people with a group of unvaccinated people under the same exposure conditions. So consider a three-month study period in which 100 out of 10,000 unvaccinated people got COVID-19. You would expect five vaccinated people to get sick in the same amount of time. That’s 5% of the 100 unvaccinated people who got sick, not 5% of the entire group of 10,000.

When people become infected after vaccination, scientists call these cases “breakthrough” infections because the virus broke through the protective barrier that the vaccine provides.

How common is a COVID-19 infection in fully vaccinated people?

Breakthrough infections are slightly more common than previously expected and are likely to increase due to the increasing dominance of the delta variant. But infections in vaccinated people are still very rare and usually cause mild or no symptoms.

For example, 46 US states and territories voluntarily reported 10,262 breakthrough infections to the US Centers for Disease Control and Prevention between January 1 and April 30, 2021. In comparison, there were a total of 11.8 million COVID-19 diagnoses during the same period.

As of May 1, 2021, the CDC stopped monitoring vaccine breakthrough cases unless they resulted in hospitalization or death. As of July 19, 2021, there were 5,914 patients with breakthrough infections from the COVID-19 vaccine who were hospitalized or died in the U.S., out of more than 159 million people nationwide who were fully vaccinated.

A study conducted between December 15, 2020 and March 31, 2021, involving 258,716 veterans who received two doses of the Pfizer or Moderna vaccine, counted 410 who developed breakthrough infections — that’s 0.16% of the total. Similarly, a New York study found 86 cases of breakthrough infections from COVID-19 between February 1 and April 30, 2021, among 126,367 people who were fully vaccinated, mostly with mRNA vaccines. This accounts for 1.2% of the total COVID-19 cases and 0.07% of the fully vaccinated population.

Even if you are fully vaccinated, you should get tested if you have any symptoms.

How serious is a breakthrough infection with COVID-19?

The CDC defines vaccine breakthrough infection as an infection in which a nasal swab can detect the SARS-CoV-2 RNA or protein more than 14 days after a person has completed the full recommended doses of an FDA-approved COVID-19 vaccine.

Note that a breakthrough infection doesn’t necessarily mean the person is feeling sick — and in fact, 27% of breakthrough cases reported to the CDC were asymptomatic. Only 10% of people with breakthrough infection were known to be hospitalized (some for reasons other than COVID-19), and 2% died. By comparison, in the spring of 2020, when vaccines were not yet available, more than 6% of confirmed infections were fatal.

In a study at US military treatment facilities, none of the breakthrough infections led to hospitalization. In another study, the vaccinated people who tested positive for COVID-19 had a quarter less virus in their bodies after just one dose of Pfizer vaccine than those who had not been vaccinated and tested positive.

What Makes a Breakthrough Infection More Likely?

Nationwide, more than 5% of COVID-19 tests come back positive on average; in Alabama, Mississippi and Oklahoma the positivity rate is above 30%. A lot of coronavirus circulating in a community increases the chance of breakthrough infections.

It is more likely in close contact situations, such as in a cramped workspace, party, restaurant, or stadium. Breakthrough infections are also more common in healthcare professionals who have frequent contact with infected patients.

For unclear reasons, nationwide CDC data showed that women are responsible for 63% of breakthrough infections. Some smaller studies also identified women as having the most breakthrough cases.

Vaccines cause a weaker immune response in the elderly and the risk of a breakthrough infection increases with age. Of the breakthrough cases tracked by the CDC, 75% occurred in patients 65 years of age and older.

Being immunocompromised or having underlying conditions such as high blood pressure, diabetes, heart disease, chronic kidney and lung disease, and cancer increase the risk of breakthrough infections and can lead to severe COVID-19. For example, recipients of fully vaccinated organ transplants were 82 times more likely to develop a breakthrough infection and had a 485 times higher risk of hospitalization and death after a breakthrough infection compared to the vaccinated general population in one study.

Vaccination is still the best choice against emerging coronavirus variants.

How do variants like delta change things?

Researchers developed today’s vaccines to ward off previous strains of the SARS-CoV-2 virus. Since then, new variants have emerged, many of which are better able to evade the antibodies produced by currently approved vaccines. Although existing vaccines are still highly effective against these variants to prevent hospitalization, they are less effective than against previous variants.

Two doses of the mRNA vaccines were only 79% effective in preventing symptomatic delta disease, compared to 89% in the case of the earlier alpha variant, according to Public Health England. A single dose was only 35% protective against delta.

About 12.5% ​​of the 229,218 cases of delta variants in England through July 19 were among fully vaccinated people.

Israel, with high vaccination rates, has reported that complete vaccination with the Pfizer vaccine could only be 39%-40.5% effective in preventing delta variant infections of any severity, compared to early estimates of 90%. Israel’s findings suggest that within six months, the efficacy of COVID-19 vaccines in preventing infection and symptomatic disease declines. The good news, however, is that the vaccine is still highly effective in protecting against hospitalization (88%) and serious illness (91.4%) caused by the now-dominant delta variant.

So how well do vaccines hold up?

As of the end of July 2021, 49.1% of the U.S. population, or just over 163 million people, has been fully vaccinated. Nearly 90% of Americans over age 65 have received at least one dose of a vaccine.

Scientists’ models suggest that vaccination saved approximately 279,000 lives in the US and prevented up to 1.25 million hospitalizations by the end of June 2021. Similarly, in England, approximately 30,300 deaths, 46,300 hospitalizations and 8.15 million infections have been prevented by COVID-19 vaccines. . In Israel, high vaccination coverage is said to have led to a 77% drop in cases and a 68% drop in hospitalizations since that country’s pandemic peak.

In the US, only 150 of the more than 18,000 deaths from COVID-19 in May were from people who were fully vaccinated. That means nearly all COVID-19 deaths in the US are among those who have not been vaccinated.

The US is “almost like two Americas,” as Anthony Fauci put it, divided between the vaccinated and the unvaccinated. Those not fully vaccinated against COVID-19 are still at risk from the coronavirus that has killed more than 600,000 people in the US so far

Written by Sanjay Mishra, PhD, Project Coordinator and Staff Scientist, Vanderbilt University Medical Center, Vanderbilt University.

Originally published on The Conversation.