Immunize.org has been refreshed! Take a tour.

Ask the Experts: COVID-19

This page was last updated on October 26, 2023. See CDC’s Use of COVID-19 Vaccines in the United States Interim Clinical Considerations for more information. For a comprehensive collection of COVID-19 tools and resources, visit Immunize.org’s Vaccines: COVID-19 main page: www.immunize.org/covid-19. For alerts about new Immunize.org or CDC COVID-19 resources, subscribe to our weekly e-newsletter, IZ Express.

Results (78)

COVID-19 is the name given to the disease caused by infection with the SARS-CoV-2 coronavirus. This virus was first detected as a cause of human illness in late 2019 in Wuhan, China, and triggered a global pandemic that began in 2020. The virus is thought to spread mainly from person to person through respiratory droplets and small particles produced when an infected person coughs, sneezes, or talks. The virus spreads easily in crowded or poorly ventilated indoor settings. Some people who are infected have no symptoms of illness, while the severity of symptomatic illness ranges from mild to life-threatening. Older adults and people of any age with underlying medical conditions are at higher risk for severe illness.

The incubation period after exposure ranges from 2–14 days, with an average of about 5 days. People with COVID-19 are generally considered potentially infectious up to 48 hours before symptom onset through 10 days after onset, though people with severe illness may be infectious longer. Symptoms may include fever or chills, cough, shortness of breath or difficulty breathing, fatigue, muscle or body aches, headache, new loss of taste or smell, sore throat, congestion or runny nose, nausea or vomiting, and diarrhea.

Last reviewed: October 26, 2023

The CDC has assembled clinical information about COVID-19 at this site: www.cdc.gov/coronavirus/2019-nCoV/hcp/index.html.

Last reviewed: October 26, 2023

CDC maintains a webpage with critical interim clinical considerations for vaccination of eligible recipients: www.cdc.gov/vaccines/covid-19/clinical-considerations/covid-19-vaccines-us.html. This covers important clinical details about COVID-19 vaccination. It is the first content to be updated soon after any announced change to CDC recommendations.

All of the CDC’s Advisory Committee on Immunization Practices (ACIP) vaccine recommendations published in MMWR can be accessed here: www.cdc.gov/vaccines/hcp/acip-recs/vacc-specific/covid-19.html.

The current recommended COVID-19 vaccination schedules for each age group are available here: www.cdc.gov/vaccines/covid-19/downloads/COVID-19-immunization-schedule-ages-6months-older.pdf.

Last reviewed: October 26, 2023

All people age 6 months and older in the United States are recommended to receive an age-appropriate updated (2023–2024 Formula) COVID-19 vaccination. Schedules vary by age and immunocompromised status. CDC’s recommended schedule for the 2023–2024 Formula for each age group is available here: www.cdc.gov/vaccines/covid-19/downloads/COVID-19-immunization-schedule-ages-6months-older.pdf.

Last reviewed: October 26, 2023

CDC recommends that all people receive an updated (2023–2024 Formula) COVID-19 vaccine at least 2 months after their most recent previous dose of COVID-19 vaccine, as authorized or approved by FDA. There are three options: Moderna or Pfizer-BioNTech mRNA vaccine (both FDA-authorized beginning at age 6 months and FDA-licensed for age 12 years and older), or Novavax adjuvanted protein subunit vaccine (authorized for age 12 years or older). Most people age 5 years and older should receive a single dose of vaccine.

Certain people need, or have the option to receive, more than one 2023–2024 COVID-19 vaccine dose:

  • All previously unvaccinated individuals age 12 years or older receiving Novavax vaccine should receive two doses separated by at least 3 weeks.
  • Children younger than age 5 years are recommended to receive at least one updated 2023–2024 Formula mRNA vaccine dose. If not previously vaccinated, or incompletely vaccinated, they should complete a 2-dose (Moderna) or 3-dose (Pfizer-BioNTech) primary series, using the same brand for all doses. CDC recommends that children younger than age 5 years receive the same brand for all doses (also referred to as homologous doses), even after completing the primary series. If it is not feasible to administer the same brand for one of the following reasons, it is acceptable to use a different brand: the brand is unavailable at the clinic at the time of the vaccination visit; the previous brand is unknown; the child would not otherwise receive a recommended vaccine; or, the child cannot continue with the previous brand due to a contraindication.
  • Individuals who are moderately or severely immunocompromised and who have not already completed a 3-dose primary series, should complete a 3-dose primary vaccination series with the updated 2023–2024 Formula. Use the same brand for all doses of the primary series, unless the brand is unavailable at the time of the clinic visit, the patient would otherwise choose not to complete the series, or the patient has a contraindication to vaccination with the original product. If the primary series is already complete, then a single dose of any age-appropriate updated (2023–2024 Formula) COVID-19 vaccine should be administered at least 2 months after the most recent dose of COVID-19 vaccine. Additional doses may be given, spaced at least 2 months apart, as determined by these individuals with their clinical teams.

For details, see CDC’s current recommended COVID-19 vaccination schedule for all people age 6 months and older here: www.cdc.gov/vaccines/covid-19/downloads/COVID-19-immunization-schedule-ages-6months-older.pdf.

Last reviewed: October 26, 2023

For individuals with moderate to severe immunocompromise, CDC states that, beyond the primary series, additional doses of 2023–2024 COVID-19 vaccine may be administered (with a minimum two-month interval) based on the clinical judgment of the individual’s healthcare provider and personal preference and circumstances.

The option to receive these additional doses is offered because vaccine effectiveness declines most rapidly in people with moderate to severe immunocompromise. Such people also have the highest risk of hospitalization with COVID-19 if infected. Although protection against severe disease is more durable than protection against milder illness, individuals in these groups may benefit from shorter intervals between doses.

See CDC’s interim immunization schedule for details: www.cdc.gov/vaccines/covid-19/downloads/COVID-19-immunization-schedule-ages-6months-older.pdf.

Last reviewed: October 26, 2023

Janssen (Johnson & Johnson) COVID-19 vaccine is no longer available for use in the United States. The last remaining doses expired May 7, 2023. People age 18 years or older who received 1 or 2 Janssen COVID-19 Vaccine doses are recommended to receive one 2023–2024 Formula dose (Moderna, Pfizer-BioNTech, or Novavax).

Last reviewed: October 26, 2023

Yes. Vaccination should be offered regardless of history of prior SARS-CoV-2 infection.

Viral testing to assess for current SARS-CoV-2 infection or serologic testing to assess for prior infection for the purposes of vaccine decision-making is not recommended. However, as with all vaccines, vaccination should be deferred until after recovery from moderate to severe illness. In addition, to minimize the risk of exposing others to SARS-CoV-2 virus, vaccination of a person diagnosed with COVID-19 generally should be deferred until the person does not pose a risk of infection to others.

Although not required, people who recently had SARS-CoV-2 infection may consider delaying a 2023–2024 COVID-19 vaccine dose by up to 3 months from symptom onset or positive test (if infection was asymptomatic). According to CDC, increasing the time between infection and vaccination may result in an improved immune response to vaccination. Among vaccine recipients at increased risk of post-vaccination myocarditis (e.g., adolescent and adult males younger than age 40), an increased interval also may reduce the rare risk of myocarditis after vaccination. A recipient’s individual risks for severe disease and current COVID-19 conditions in the community should be taken into account when deciding whether to delay vaccination up to 3 months after infection.

Last reviewed: October 26, 2023

CDC defines an “additional primary dose” as a subsequent dose of vaccine administered to people who are less likely to develop a protective immune response after initial vaccination because of moderate or severe immunocompromise. All previously unvaccinated people with moderate or severe immunocompromise are recommended to receive an initial 3-dose primary COVID-19 series.

CDC defines a “booster dose” as a subsequent dose of vaccine administered to enhance or restore protection which might have declined over time after primary series vaccination.

Last reviewed: October 26, 2023

Under section 564 of the Federal Food, Drug, and Cosmetic Act (FD&C Act), the FDA Commissioner may allow unapproved medical products or unapproved uses of approved medical products to be used in an emergency to diagnose, treat, or prevent serious or life-threatening diseases or conditions caused by chemical, biological, radiological or nuclear threat agents when there are no adequate, approved, and available alternatives. Vaccines that receive an EUA may later be fully licensed by the FDA.

Last reviewed: October 26, 2023

No. Only the updated (2023–2024 Formula) COVID-19 vaccines are currently authorized or approved for use in the United States.

Last reviewed: October 26, 2023

Moderna COVID-19 Vaccine has two updated (2023–2024 Formula) presentations. CDC has summarized basic information about the different presentations and their storage, preparation, and administration in its Moderna COVID-19 vaccine at-a-glance document available here: www.cdc.gov/vaccines/covid-19/info-by-product/moderna/downloads/vaccine-at-a-glance.pdf.

Vaccinators who stock multiple presentations should check three times to confirm that they are administering the proper product with the proper dose and dose volume for each patient.

Last reviewed: October 26, 2023

Both the Pfizer-BioNTech COVID-19 Vaccine and the Moderna COVID-19 Vaccine are lipid nanoparticle-formulated, nucleoside-modified mRNA vaccines encoding the prefusion spike glycoprotein of SARS-CoV-2, the virus that causes COVID-19. The 2023–2024 formula targets the Omicron XBB.1.5 sublineage of the spike protein. Visit this CDC website for more details about how mRNA vaccines work: https://www.cdc.gov/coronavirus/2019-ncov/vaccines/different-vaccines/how-they-work.html.

Last reviewed: October 26, 2023

The original COVID-19 vaccines all targeted the spike protein of the original SARS-CoV-2 virus. The current vaccines target the spike protein of a more recently circulating strain, known as the Omicron XBB.1.5 sublineage. The update is intended to boost production of antibodies that protect more effectively against disease caused by currently circulating Omicron subvariants.

The process of updating the strain without changing anything else is similar to the seasonal strain changes made for influenza vaccinations each year; FDA does not require manufacturers to repeat the large-scale clinical trials necessary for the original products before authorizing updated vaccines. Vaccine safety, side effects, and risk of allergic reactions are expected to be comparable to the original vaccines of the same brand and dose. As with seasonal influenza vaccines, future COVID-19 vaccines can continue to be updated when needed, as the circulating viruses evolve.

Last reviewed: October 26, 2023

No. The last U.S. doses of Janssen COVID-19 Vaccine expired May 7, 2023.

Last reviewed: October 26, 2023

Novavax COVID-19 Vaccine, Adjuvanted contains the Omicron XBB.1.5 subvariant SARS-CoV-2 spike protein and Matrix-M adjuvant. The saponin-based adjuvant is made from extracts of the bark of the Soapbark tree native to Chile. It is added to enhance the immune response of the vaccine recipient. The spike protein is produced in insect cells.

It is authorized for emergency use in people age 12 years or older as a two-dose primary series for previously unvaccinated individuals (whether or not they are immunocompromised), with the doses given at least 3 to 8 weeks apart. Individuals who have had one or more doses of any previous authorized or approved COVID-19 vaccine formulation are recommended to receive only one dose of the current formulation. Individuals with moderate or severe immunocompromise may receive additional doses as determined by their healthcare team, based on their specific circumstances.

Last reviewed: October 26, 2023

The effectiveness of Novavax COVID-19 Vaccine against currently circulating variants of SARS-CoV-2 is not precisely known. Clinical trials of this vaccine were conducted against earlier variants no longer in circulation; however, the vaccine showed very good effectiveness against moderate to severe disease during clinical trials. Testing conducted on antibodies in the serum of Novavax vaccine recipients suggests that vaccination provides substantial protection against severe disease and death caused by currently circulating variants, consistent with other available COVID-19 vaccines.

Last reviewed: October 26, 2023

CDC has developed a “one-stop” product-specific webpage for each COVID-19 vaccine with all of the important details that immunization providers need to know: www.cdc.gov/vaccines/covid-19/info-by-product/index.html.

Last reviewed: October 26, 2023

All children age 6 months and older should be vaccinated against COVID-19 with at least one dose of an updated (2023–2024) formulation vaccine. COVID-19 vaccination of children in this age group has been demonstrated to be safe and to prevent hospitalization and severe complications of COVID-19 illness. Both Pfizer-BioNTech and Moderna mRNA products are authorized for use in children down to 6 months of age. Novavax adjuvanted protein subunit vaccine is authorized for use beginning at age 12 years.

Most children younger than age 5 years initially require a primary series of two (Moderna) or three (Pfizer-BioNTech) doses of the same brand. The primary series may include doses of previously authorized formulations. CDC recommends use of the same brand (referred to as homologous doses) for all recommended doses given to children younger than age 5 years. If it is not possible to administer a homologous dose (the brand is unavailable at the time and location of the vaccination visit, the brand of a previous dose is unknown, the child would not be vaccinated with the homologous dose due to a contraindication or other reason), then administer the age-appropriate formulation of the available brand. A 3-dose primary series is required for all children and adults who have moderate or severe immunocompromise. Refer to CDC’s interim schedule for details: www.cdc.gov/vaccines/covid-19/downloads/COVID-19-immunization-schedule-ages-6months-older.pdf.

CDC has also prepared infographics to make it easier to determine exactly what vaccination is due for each patient, based upon the patient’s age, vaccination history, and immunocompromised status. For most children younger than age 12 years, see this CDC infographic: www.cdc.gov/vaccines/covid-19/downloads/COVID19-vaccination-recommendations-most-people.pdf.

For children younger than 12 years who are moderately or severely immunocompromised, see this CDC infographic: www.cdc.gov/vaccines/covid-19/downloads/COVID19-vaccination-recommendations-immunocompromised.pdf.

Last reviewed: October 26, 2023

This child should receive the dose recommended for his age at the time of the vaccination visit. At age 11 years, an age-appropriate single dose of either Pfizer-BioNTech or Moderna mRNA vaccine is recommended. If the patient arrives in your clinic after turning 12 years old, the 2023-2024 Formula Novavax protein subunit vaccine is also an option. If using the Novavax product, a previously unvaccinated person requires two doses, given 3 to 8 weeks apart, as a primary series.

Last reviewed: October 26, 2023

This page was updated on .

Our Affiliated Sites

For the Public: VaccineInformation.org
For Coalitions: ImmunizationCoalitions.org
65+ Flu Defense
Protect vulnerable, older adult patients from influenza.
Give2MenACWY.org
Improve protection from meningococcal serogroups ACWY.
Mass Vaccination Resources
Access guidance documents, toolkits, and helpful resources.
NAIIS
Home of the National Adult and Influenza Immunization Summit.