Breakthrough infections of COVID-19 are possible, but relatively rare, in people who have been fully vaccinated against the disease. Some scientists estimate that breakthrough infections of COVID-19 may be as uncommon as 1 out of 5,000-10,000 vaccinated people, based on numbers in the United States. More research is being done, as the US Centers for Disease Control and Prevention (CDC) focused limited resources on tracking symptomatic breakthrough cases rather than all incidences of breakthrough infections.

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Many preventative interventions that we use in our daily lives do not offer 100% protection, including vaccines and birth control. Although the approved COVID-19 vaccines are highly effective, breakthrough infections are expected. As a higher number of people get vaccinated, it’s statistically possible that a higher number of breakthrough infections would occur.

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Many factors can contribute to an increase in breakthrough infections and the appearance of waning immunity, including the higher transmissibility of new variants of concern (VOCs) and the lifting of public health restrictions. The people who received COVID-19 vaccine doses earlier were generally prioritized for being at higher risk of getting COVID-19, such as frontline medical workers, so there could be different explanations for a higher rate of breakthrough infections among this population.

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Breakthrough infections after vaccination tend to have milder symptoms, shorter recovery times, and lower transmissibility rates. This suggests that even when the body’s defenses do not prevent infection or onset of disease, the immune system can still generate important protective responses. Current reports of breakthrough infections do not necessarily mean that immunity to COVID-19 is waning significantly.

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Waning immunity refers to when the immune system’s protective responses, developed through vaccination or natural infection, begin to decline over time. This happens with many diseases, so it can sometimes be beneficial for people to receive vaccine booster doses. For example, booster shots are recommended for the diphtheria-tetanus-acellular pertussis (DTaP) vaccine. Immunity can wane gradually, so vaccines with waning immunity can still be highly protective for months or years.

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In fact, vaccines can continue to offer protection even after the vaccine-generated antibodies begin to decline in number or become undetectable. This is relevant because research published in the Lancet found that antibody levels can start declining within a few months after COVID-19 vaccination. It is natural and expected for antibody levels to decline over time with various diseases, as this allows the body to conserve and prioritize limited energy and resources.

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Waning protection from antibodies should not be interpreted as the body’s immune defenses having disappeared completely, because there are other immune responses that can be durable for longer periods of time. Beyond immunity from antibodies, there is cell-mediated immunity that comes from the immune system’s T-cells.

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T-cells can hide out in the body’s tissues for months or even years, emerging to attack foreign invaders like the viral particles that cause COVID-19. T-cells could better defend against variants because of their ability to attack with less specificity than antibodies. Protection from T-cells has been detected for as long as 11 years with severe acute respiratory syndrome (SARS), and for over 6 months with COVID-19, with more studies ongoing.

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Scientists are gathering more data on breakthrough infections and the body’s immune responses to help determine how long protection against COVID-19 lasts. For example, peer-reviewed research published in Nature found persistent immune activity in the body 4+ months after COVID-19 vaccination. Several pharmaceutical companies have conducted studies showing that COVID-19 vaccines can protect against hospitalization and death for at least half a year. More research is needed to understand COVID-19 immunity in the long-term.

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With more data on the speed and extent of waning immunity for COVID-19, scientists will be able to make better decisions about whether and when vaccine booster doses might be needed. This may depend on the context around each patient, including which vaccine was received, and the recipient’s health condition. For example, studies are being done on providing supplemental vaccine doses to people who received a single-dose COVID-19 vaccine. Additionally, in several countries with more plentiful vaccine resources, people who are immunocompromised or at high-risk in other ways have already started receiving additional COVID-19 vaccine doses.

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While there is an ongoing scientific debate about COVID-19 vaccine booster doses, particularly in the context of limited global resources for vaccination, many scientists agree that there needs to be clear evidence of significant benefit from booster shots before recommending them to the general population.

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Current data suggest that despite the Delta variant and other variants of concern, the initial full doses of approved vaccines continue to protect a high percent of people from severe disease, hospitalization, and death. This situation may evolve with more research results on the duration of immunity against COVID-19, the progression of the pandemic, and the emergence of new variants.

As more countries begin considering and offering vaccine booster doses for COVID-19, confusion as well as misinformation have been spreading around how fast the protection from vaccination may decline. False information about the vaccines being ineffective or losing effectiveness quickly can cause unnecessary fear among vaccinated people and contribute to vaccine hesitancy among unvaccinated people. While more research is being done into how long protection lasts from the COVID-19 vaccines, scientists agree that vaccination saves lives by effectively protecting against severe symptoms, hospitalization, and death for multiple months at minimum.

Research into the need for COVID-19 booster shots is also happening in the context of vast inequities in vaccine access around the world. The World Health Organization (WHO) has asked nations to reconsider giving booster shots to fully-vaccinated healthy people at this time, when large proportions of the global population lack access to initial doses of approved COVID-19 vaccines. Policies around booster shots need to be informed by research, rather than politics or alarmism from misinformation, so scientists are working to better understand what waning immunity from COVID-19 vaccines looks like and its implications. For example, the US Food and Drug Administration (FDA) has put out neutral statements ahead of the September 17, 2021 public review of data around booster doses for the Pfizer COVID-19 vaccine. The government of the Philippines announced preparations to budget for booster shots in 2022 while waiting for the evaluation and recommendations by national and international health agencies.