Is the COVID-19 Vaccine Effective Against the Delta Variant?

Alexis Bryan
Alexis Bryan23 Aug 2022

More infectious strains of the coronavirus, such as the Delta variant, threaten protection from the currently approved vaccines. At this time, currently authorized vaccines are working. COVID-19 vaccination prevents severe illness, hospitalization, and death from the SARS-CoV-2 virus, but breakthrough cases are expected. 

Especially with the Delta variant circulating, breakthrough cases are expected. The best way to protect yourself against serious illness is still through vaccination. If you live or work in a substantial or high transmission area, getting vaccinated is even more important. Our team at Mira can help you find an appointment today.

What We Know about COVID-19 Vaccine Breakthrough Cases

COVID-19 vaccine breakthrough cases refer to those who are fully vaccinated yet test positive for COVID-19 cases. Those who are immunocompromised or over 65 years old are most at risk for breakthrough cases. 

Recently we have seen an increase in positive COVID-19 tests among those who have been fully vaccinated. The CDC is monitoring the situation to provide the best guidance on booster shots and public health protocols. COVID-19 vaccine breakthrough cases occur for two reasons:

  1. Increased infectiousness of variants
  2. Waning immunity from vaccination

The CDC has summarized the most important things to know about COVID-19 Vaccine Breakthrough Cases:

  • Breakthrough infections are expected. COVID-19 vaccines are effective at preventing most infections. However, like most vaccines, they are not 100% effective.
  • Fully vaccinated people with a breakthrough infection are less likely to develop serious illness than those who are unvaccinated and get COVID-19.
  • Even when fully vaccinated people develop symptoms, they tend to be less severe symptoms than unvaccinated people. This means they are much less likely to be hospitalized or die than people who are not vaccinated.
  • People who get vaccine breakthrough infections can be contagious.

COVID-19 Vaccine Breakthrough Case Infectiousness

A new study from the Netherlands was released last week suggesting that fully vaccinated people breakthrough cases are less infectious than COVID-19 cases among unvaccinated people. 68.6% of breakthrough infections tested positive compared with 84.9% of infections in unvaccinated patients. While vaccinated people can still become infected with COVID-19, they shed less virus, meaning they may be less likely to spread the virus.

COVID-19 Testing for Fully Vaccinated Individuals

To start, fully vaccinated means you have received both doses of an mRNA vaccine (which take two weeks to take full effect) or one dose of another approved vaccine. If you have been exposed to COVID-19, you should get tested 3-5 days after exposure whether or not you have symptoms. You should also get tested if you experience any flu-like symptoms to ensure it is not COVID-19. If possible, a PCR test is always recommended.

Getting tested even when you are vaccinated is important since you can still spread the virus. Given there is evidence that immunity from vaccination wanes, you never know who is vulnerable to infection around you. Especially since kids under 12 cannot yet get vaccinated, it is important to protect those around you.

COVID-19 Symptoms for Fully Vaccinated Individuals

COVID-19 vaccine breakthrough cases most often resemble a cold and can be resolved at home. Fully vaccinated individuals are extremely likely to have an asymptomatic infection after testing positive. If you do experience symptoms, it is very rare they will be serious enough to need hospitalization.

COVID-19 Vaccine Breakthrough Cases and Variants

Current data suggest that COVID-19 vaccines authorized for use in the United States offer protection against most SARS-CoV-2 variants circulating in the United States. Studies looking at the effectiveness of COVID-19 variants are currently being conducted among healthcare workers and the general population. The table below summarizes what we know so far.

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Vaccine Effectiveness by Variant

VariantEffectiveness of Vaccines
Alpha - B.1.1.7


Currently, authorized vaccines do work against this variant. Some breakthrough infections in fully vaccinated people are expected but remain rare. All vaccines are particularly effective against severe illness, hospitalization, and death.

Beta - B.1.351


Currently, authorized vaccines do work against this variant. Some breakthrough infections are expected but remain rare. All vaccines are particularly effective against severe illness, hospitalization, and death.

Gamma - P.1


Currently, authorized vaccines do work against this variant. Some breakthrough infections are expected, but remain rare. All vaccines are particularly effective against severe illness, hospitalization, and death.

Delta - B.1.617.2


Infections happen in only a small proportion of fully vaccinated people, even with the Delta variant. Some breakthrough infections are expected but remain rare. However, preliminary evidence suggests that fully vaccinated people who do become infected with the Delta variant can spread the virus to others. All vaccines are particularly effective against severe illness, hospitalization, and death.

Additional information about mRNA vaccines against the Delta variant:

  • The Pfizer and Moderna mRNA vaccines seem less effective against confirmed infection and symptomatic infection from the Delta variant compared to the Alpha variant.
  • Despite a decrease in protection from infection, the rate of hospitalization from Delta is comparable to that of Alpha.

U.S. Vaccination Update

As of August 29, 2021, 173.5 million people in the United States have been fully vaccinated, amounting to 368 million doses administered. This has been a vaccination campaign of unprecedented size.

Vaccination Data by Age Group

People VaccinatedAt Least One DoseFully Vaccinated
% of Total Population61.6%52.3%
Population ≥ 12 Years of Age204,207,795173,388,637
% of Population ≥ 12 Years of Age72%61.2%
Population ≥ 18 Years of Age191,142,250163,404,705
% of Population ≥ 18 Years of Age74%63.3%
Population ≥ 65 Years of Age50,301,73444,682,670
% of Population ≥ 65 Years of Age92%81.7%

Booster Shots for Immunocompromised People

On August 13th, 2021, CDC recommended that moderate to severely immunocompromised people receive an additional dose of an mRNA COVID-19 vaccine. Since then, 901,000 people have received an additional dose. This ensures sufficient protection against COVID-19 for people whose immunity has likely waned overtime at a disproportionate rate than those who are not immunocompromised.

People who are moderate to severely immunocompromised are especially vulnerable to COVID-19 infection and may experience more serious infection. One main reason is that people who are immunocompromised may not build the same level of immunity from vaccination, leaving them unprotected despite getting vaccinated. Those who are recommended to receive a booster at this time include people who:

  • Have been receiving active cancer treatment for tumors or cancers of the blood
  • Received an organ transplant and are taking medicine to suppress the immune system
  • Received a stem cell transplant within the last 2 years or are taking medicine to suppress the immune system
  • Moderate or severe primary immunodeficiency (such as DiGeorge syndrome, Wiskott-Aldrich syndrome)
  • Advanced or untreated HIV infection
  • Active treatment with high-dose corticosteroids or other drugs that may suppress your immune response

COVID-19 Vaccine Breakthrough Cases Frequently Asked Questions (FAQs)

As cases rise and the pandemic continues, it can be difficult to stay up to date with the current situation and recommendations. Below we answer a few questions about breakthrough cases, the state of the pandemic, and booster shots.

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Who is most at risk for COVID-19 breakthrough cases?

Anyone who has less immunity against COVID-19 is more at risk. This includes people who are immunocompromised, over 65 years old or have comorbidities like diabetes or obesity. It is not always clear who is most protected from vaccination because recent research has shown waning immunity among vaccinated people.

Are SARS-CoV-2 virus mutations behind breakthrough cases?

As mentioned, new variants can be more contagious. The SARS-CoV-2 virus also can mutate into a vaccine-resistant strain, but this is not the case at this time. The vaccines currently available are effective against all known variants. 

The main concern regarding COVID-19 variants is the possibility of new ones emerging due to low rates of vaccination. COVID-19 spreads more rapidly among those who are unvaccinated as the pandemic has now been referred to as a pandemic of the unvaccinated.

Where are areas with high or substantial transmission?

The CDC tracks community transmission in the United States by county. Over the past seven days, 93.79% of the U.S. is classified as an area of high transmission. The risk of transmission is determined by two statistics: the number of new cases per 100,000 people and the percentage of positive tests over the past seven days. To protect yourself against COVID-19 infections in these areas, wearing a mask is recommended.

What more should I know about booster shots at this time?

CDC recommends the additional dose of the same mRNA COVID-19 vaccine be administered at least four weeks after the second dose of the Pfizer-BioNTech COVID-19 vaccine or Moderna COVID-19 vaccine. A person should not receive more than three mRNA vaccine doses.

There is not enough data to determine whether an additional dose would be beneficial for immunocompromised people who received the Johnson & Johnson’s Janssen COVID-19 vaccine. The idea of a booster shot is relatively new, so there is not much information about the potential risks and safety yet. If you are considering getting an additional dose and are worried, consult with your doctor beforehand.

Bottom Line

Even if the vaccines are working as expected, COVID-19 vaccine breakthrough cases will occur. While it is not yet time to put away your mask, the WHO recently reported the number of new cases worldwide "seems to be plateauing" after increasing for nearly two months.

To sum up all of the recommendations mentioned above: get vaccinated if you are not. If you are already vaccinated, wear a mask indoors. All of the data we have so far support this as the best protocol to combat the pandemic. We have the power to reduce the burden of COVID-19 on the healthcare system and global economy through these simple measures.

Alexis Bryan

Alexis Bryan MPH, is a recent graduate of Columbia’s Mailman School of Public Health. She is passionate about increasing access to care to improve health outcomes. Outside of work, she loves to travel, read, and pay too much attention to her plants.