Update: The Centers for Disease Control and Prevention (CDC) currently recommends people aged 65 and older, long-term care residents, people aged 50-64 years with certain underlying medical conditions, and people aged 18-64 years who are at increased risk for COVID-19 exposure and transmission because of occupational or institutional setting receive booster shots of Pfizer-BioNTech’s COVID-19 vaccine at least 6 months after completing the primary vaccine series. Read the full statement here.
The United States is experiencing a rise in the number of COVID-19 cases, hospitalizations, and deaths, driven by the Delta variant of concern (B.1.617.2; VOC) that was first identified in 2020 in India. The Delta variant of SARS-CoV-2, the virus that causes COVID-19, is more transmissible and now the most dominant strain in the U.S., with the number of new cases in all 50 states up over the past 2 weeks.
Low Vaccine Uptake Driving Outbreaks
According to U.S. health officials, most of the outbreaks are occurring in areas of the country that have low vaccination coverage because unvaccinated or partially vaccinated people are most at risk of infection. These pockets of lower vaccination coverage can provide the perfect conditions for transmission, especially in the absence of other prevention measures, requirements for which have been lifted in most areas.
However, some vaccinated individuals are being diagnosed with COVID-19. These “breakthrough” infections are still rare, and researchers continue to evaluate how well the currently authorized vaccines protect against Delta and other SARS-CoV-2 variants.
Vaccines vs. Delta
Getting full vaccination is important, as data show only one dose of two-dose regimens (Pfizer-BioNTech and Moderna) does not produce a strong immune response to the Delta variant.
So far, studies show one of the most widely used vaccines, the mRNA vaccine from Pfizer-BioNTech, offers strong protection against severe disease, hospitalization, and death caused by infection with Delta, although protection may be reduced for preventing symptomatic disease.
- Preprint (not peer-reviewed) data from Pfizer-BioNTech shows the efficacy of its vaccine to prevent symptomatic COVID-19 declined about 6% every 2 months over a period of 6 months. From 7 days to <2 months post-second dose, vaccine efficacy was 96.2%; from 2 months to <4 months, efficacy was 90.1%; and from 4 months to 6 months, efficacy was 83.7%. Overall, vaccine efficacy was 91.1% for preventing COVID-19, and 96.7% against severe disease.
- A study from Public Health England showed the Pfizer-BioNTech vaccine is 88% effective at preventing symptomatic disease and 96% effective against hospitalization with the Delta variant. The vaccine was about 80% effective at preventing infection with the variant. Earlier real-world data from England found one dose of the vaccine was only 33% effective at preventing symptomatic disease, underlining the importance of receiving both doses.
- A study conducted in Scotland showed two doses of the Pfizer-BioNTech vaccine were effective in reducing the risk of infection with the Delta variant by 79%, but the effect on preventing infection appeared to be lessened when compared with an earlier variant. The vaccine also was effective at reducing the risk of COVID-19-related hospitalization in people with the Delta variant.
- According to preliminary data from Canada, full vaccination with the Pfizer-BioNTech shot was about 87% effective at preventing infection with the Delta variant.
- Preliminary data from Israel show full vaccination with the Pfizer-BioNTech was 64% effective in preventing infection with and symptomatic illness caused by the Delta variant. This rate is lower than the vaccine’s effectiveness prior to circulation of the Delta variant in Israel (95%), but the data show two doses are 93% effective in preventing serious illness and hospitalization.
The vaccine from Moderna, also an mRNA vaccine, offers similar protection against the Delta variant and other variants tested, according to data from a lab study announced by the company. In the same Canadian study as noted above, researchers found the Moderna vaccine was 72% effective at preventing infection with the Delta variant after only one dose, but there were not enough data to calculate protection after two doses. Based on this information, scientists believe protection from two doses of Moderna vaccine are expected to be higher for infection, symptomatic disease, and hospitalization.
Johnson & Johnson (J&J, which produces the Janssen vaccine) recently announced preliminary data demonstrating its single-shot J&J viral vector vaccine generates a strong immune response for at least eight months against all highly prevalent variants, including Delta. A previous study found the J&J vaccine to be nearly 77% effective against severe COVID-19 fourteen days after administration, with that protection rising to 92.4% 42 days post-vaccination. However, the latter study did not examine protection against the Delta variant.
Booster Shots Needed?
Vaccine booster shots are additional vaccine doses meant to increase a person’s immune response to protect against infection or severe disease. The effects of some vaccines might wear off over time, requiring an extra shot, or boosters might be specifically designed to target new virus variants.
Experts generally agree that the authorized SARS-CoV-2 vaccines work well in healthy people to produce a long-lasting immune response, including to viral variants. However, U.S. health officials have suggested booster shots might be necessary for some older individuals or those with weakened immune systems because they do not generate a strong response to the initial shot(s). People with weakened immune systems make up about 2.7% of the U.S. population and include those who are transplant patients, on cancer chemotherapy, have autoimmune diseases, or on immunosuppressant drug regimens. The other reason a booster dose might be needed is to provide protection against a new variant. However, variant-specific booster shots have not yet been fully tested.
All of the vaccine makers with shots authorized in the U.S. say their data suggest people eventually will need an additional dose following their first complete series of vaccinations. According to preliminary unpublished data from Pfizer-BioNTech, a third dose of its vaccine produced a five-fold increase in antibodies against the Delta variant among people ages 18 to 55 after their second dose and an 11-fold increase among people between the ages of 65 and 85. Both Pfizer-BioNTech and Moderna have plans to ask the U.S. Food and Drug Administration (FDA) to authorize booster doses.
Typically, health officials cannot recommend booster shots until the FDA authorizes additional doses for any of the vaccines or grants full approval of a vaccine. But an official from Centers for Disease Control and Prevention (CDC) recently said the agency is looking into ways to recommend additional doses for some individuals before the FDA makes any regulatory decisions. On July 8, the CDC and the FDA released a joint statement saying the agencies “are prepared for booster doses if and when the science demonstrates that they are needed.” The key to providing anyone with booster doses is to first determine whether they are safe and effective through further testing, including clinical trials.
With only about 50% of the U.S. population fully vaccinated against COVID-19, the most important use of vaccine supplies is to provide initial coverage to those who qualify.
Key Lessons
- All of the vaccines authorized in the U.S. are highly effective at preventing symptomatic COVID-19 infection, hospitalization, and severe disease, including cases caused by the Delta variant.
- Having a full regimen of a two-dose vaccine (Pfizer-BioNTech or Moderna) is more effective against the Delta variant than receiving only one dose.
- At this time, booster shots are not recommended for any of the authorized vaccines in the U.S.
- The best ways to prevent COVID-19include getting vaccinated and avoiding exposure to the virus. Even if you are vaccinated, mask wearing and physically distancing continue to be effective tools for reducing the risk of virus transmission, especially in indoor areas, in communities with low vaccination rates, and for individuals who are ineligible for or do not have access to vaccines. This is especially important in areas with high risk of transmission.
How You Can Help
COVID-19 vaccines are free and widely available. If you are looking for a place to receive a COVID-19 vaccine, you can visit Vaccines.gov, where you can enter your ZIP code to search for available vaccination sites near you.
Post-vaccination studies such as HERO-TOGETHER are an important way to help researchers learn more about COVID-19 vaccines. If you have recently received a COVID-19 vaccine, please join us in HERO-TOGETHER. It takes less than 10 minutes to sign up and is an easy way to continue to fight COVID-19. You will help our understanding of the long-term outcomes of the authorized COVID-19 vaccines and receive up to $200 compensation and information about what we are learning.