Preguntas sobre la vacunación contra el COVID-19

¿Se pregunta qué vacuna debe recibir, cuándo y por qué? Encuentre respuestas a esas preguntas y más.

¿Qué vacunas se están usando en los EE. UU.?

For most of the pandemic three vaccines(link opens in new window), commonly referred to by the names of the companies that make them, have been available in the United States:

The Pfizer and Moderna vaccines are preferred, while the Johnson & Johnson vaccine is recommended only in some cases(link opens in new window).

A fourth vaccine, made by Novavax, received Food and Drug Administration authorization as a two-dose series in people ages 18 and older in the U.S. in July 2022. That vaccine has also received the support of the Centers for Disease Control and Prevention and is expected to be available soon.

¿Cómo funcionan las vacunas?

The Pfizer and Moderna vaccines are a type called a messenger RNA vaccine(link opens in new window). Johnson & Johnson's is a viral vector vaccine(link opens in new window), and Novavax is a protein-based (or protein-subunit) vaccine.(link opens in new window) All these vaccines teach the body how to fight the virus that causes COVID-19 without you actually having to get the illness. None of the vaccines causes the illness — there is no live virus in any of the vaccines.

Research shows the vaccines can help protect you from getting COVID-19, although fully vaccinated people can get infected. (That’s called a “breakthrough infection(link opens in new window).”) But more importantly, vaccination has been shown to greatly reduce the risk of getting seriously ill and being hospitalized if you do get COVID-19.

I’m not vaccinated yet. Where do I start?

Vaccine availability, dose and recommended number of shots vary depending on the type of vaccine you get and your age and immune status.

The first round of COVID-19 shots is called your “primary series” of vaccinations. If you’re an adult, in most cases you would start with two doses of the Pfizer vaccine (three to eight weeks apart) or two doses of the Moderna vaccine (four to eight weeks apart). For the primary series, experts recommend you get the same brand of shot each time. Your health care team can guide you on whether it’s best to get the second shot as soon as you’re eligible (particular for people 65 and older) or to wait the full eight weeks for the second vaccination.

Some people require three doses for their primary series — including children ages 6 months through 4 years receiving the Pfizer vaccine and people of any age with markedly weakened immune systems receiving the Pfizer or Moderna shots.

The Johnson & Johnson Janssen is not recommended except in unusual cases — for instance, if you’re allergic to any ingredients in the Pfizer or Moderna vaccines or if those vaccines are not available to you. The primary series is one dose.

The CDC provides details(link opens in new window) about the vaccination schedule for each COVID-19 vaccine used in the U.S. Visit www.vaccines.gov(link opens in new window) to find the vaccination you (or your family members) need near you.

¿Cuánto tiempo me protegerá la vacuna?

Scientists are studying this question, although research has indicated that the level of protection wanes over time — leading public health authorities to recommend that everyone 18 and older who has been vaccinated get a booster shot(link opens in new window). Second boosters are advised for all people 50 and older as well as adults and some children with weakened immune systems.

When should I get a first or second booster shot? Which vaccine should I get?

If you’re over age 18 and the Pfizer or Moderna vaccine was used for your primary series, you can receive booster vaccinations using either of those two vaccine types. For most adults, that first booster should be five or more months after you complete your primary series.

If you received the Johnson & Johnson vaccine initially, a first booster shot with the Pfizer or Moderna vaccine is advised for most people two months after your original shot.

Second booster shots, using the Pfizer or Moderna vaccines, are recommended at least four months after the first booster for adults 50 and over and adults with moderately or severely weakened immune system. A booster shot also is recommended for children 5 and older who received the Pfizer vaccine in their primary series.

Because recommendations for booster shots are tailored for children and adults based on age, previous shots received and immune system status, the CDC has a booster tool(link opens in new window) to help guide you. If you have questions about the vaccines, consult your health care team.

How do I know if I’m ‘immunocompromised’?

Conditions such as advanced HIV infection and some genetic disorders can compromise, or weaken, the immune system. So can treatments including stem cell transplantation, some anti-cancer medicines, high-dose corticosteroids and immune-suppressing drugs used after organ transplantation.

Your health care team can guide you on whether you have a weakened immune system and should receive additional vaccinations. Also, continue other COVID-19 precautions including masking, distancing and frequent handwashing, and be sure people close to you are vaccinated whenever possible.

Si va a recibir dosis adicionales de la vacuna, no olvide llevar su tarjeta de Registro de vacunación contra el COVID-19 original de los CDC. Al igual que con la serie de vacunación original, las vacunas adicionales son gratuitas para todas las personas en los EE. UU.

What does ‘fully vaccinated’ mean? When am I ‘up to date’?

The CDC considers people fully vaccinated two weeks after receiving the second dose in a two-dose primary series (Pfizer or Moderna) or the initial single dose of the Johnson & Johnson Janssen vaccine. Up to date means they’ve received their primary vaccination series and all recommended boosters when they are eligible for them.

¿Qué es la "autorización de uso de emergencia"? ¿En qué se diferencia de la “aprobación” de la FDA?

All the vaccines available in the U.S. have initially received an official designation called emergency use authorization from the Food and Drug Administration. During a public health emergency such as the COVID-19 pandemic, the FDA can allow the use of medical products such as the new vaccines that haven't yet received full agency approval if they are needed to fight serious or life-threatening disease and no acceptable alternatives are available. In evaluating the evidence for an emergency use authorization, the agency must decide that the known and potential benefits outweigh the known and potential risks of the medical product in question.

For full approval, the COVID-19 vaccines must clear the FDA's standard, more comprehensive review process for quality, safety and effectiveness, which builds on the extensive data previously submitted for the emergency use authorization.

In August 2021, the primary series of the Pfizer vaccine became the first fully approved COVID-19 immunization for people ages 16 and older; in July 2022 it was additionally approved for children ages 12 to 15. In January 2022, the Moderna vaccine received full approval for primary vaccination in people 18 and older. These vaccines also have emergency use authorization for other groups. The Johnson & Johnson Janssen vaccine is available (on a more limited basis(link opens in new window)) under emergency use authorization.

In July 2022, the FDA issued(link opens in new window) emergency use authorization for a fourth vaccine, Novavax, to prevent COVID-19 in people 18 and older.

Me enteré de que había inquietudes sobre la vacuna de Johnson & Johnson contra el COVID-19. ¿Qué son?

Two serious medical conditions have been associated with the J&J vaccine – a neurological condition called Guillain-Barré syndrome and a clotting disorder called thrombosis-thrombocytopenia syndrome.

In Guillain-Barré, the immune system attacks nerve calls, causing muscle weakness or sometimes paralysis. Other symptoms include difficulty with eye muscles and vision; trouble swallowing, speaking or chewing; "pins and needles" sensation in the hands and feet; and coordination problems.

Although a rare occurrence, Guillain-Barré has been diagnosed in patients more frequently in the wake of vaccination with the J&J vaccine, compared with the Pfizer or Moderna shots. Most people fully recover from Guillain-Barré, the CDC reports.

The other condition, thrombosis-thrombocytopenia syndrome (TTS), is linked to about four cases per every million doses given of the J&J vaccine. TTS is a potentially deadly disorder that involves clotting, or thrombosis, in the cerebral venous sinuses (where it can prevent blood from draining out of the brain and cause a stroke), or in other sites including the large blood vessels of the abdomen and veins in the legs. This clotting is counterintuitively combined with thrombocytopenia, or low levels of blood platelets, which help blood clot.

People who get the J&J vaccine should seek medical care right away if they have any of these symptoms afterward:

  • Respiración entrecortada
  • Dolor torácico
  • Dolor en las piernas o inflamación
  • Dolor abdominal o de espalda persistente
  • Jaquecas persistentes o graves o visión borrosa
  • Hematomas o pequeñas manchas de sangre debajo de la piel fuera del lugar de la inyección
  • Náuseas y vómitos

Estos síntomas son diferentes de los síntomas similares a los de la gripe, como la fiebre, que son comunes después de la vacunación.

¿Estas afecciones son un problema con las otras vacunas?

Neither of these conditions, TTS or Guillain-Barré, has been associated with the Pfizer and Moderna vaccines — and about 580 million doses of those have been given so far. However, there have been rare reports of inflammatory conditions called myocarditis and pericarditis(link opens in new window) with these vaccines (see "side effects" question below). Research shows the benefits of COVID-19 vaccination outweigh the low risk of myocarditis after receiving the vaccine.

¿Existen efectos secundarios graves de la vacuna?

Muy rara vez puede ocurrir una reacción alérgica grave y peligrosa llamada anafilaxia poco después de aplicar la vacuna. Los síntomas de anafilaxia incluyen garganta inflamada, erupción que puede causar picazón, sibilancias u otros problemas para respirar y desmayo. Si cree que tiene esa reacción grave después de salir del sitio de vacunación, llame inmediatamente al número de emergencias local.

Experts have estimated anaphylaxis has occurred in about 5 cases per million vaccinated in the U.S. These reactions most commonly happen within 15 minutes of when the shot was given — that's why there's a short waiting period, with health care workers standing by if treatment is needed, after your vaccination.

You could also have a less severe allergic reaction within four hours of your shot, with symptoms such as hives, swelling and wheezing or difficulty breathing. If you have any of these allergic reactions, the CDC recommends(link opens in new window) you not get a second shot of that type of vaccine.

In addition, public health experts are monitoring(link opens in new window) vaccine safety and side effect data for cases of rare heart-related complications, myocarditis or pericarditis, after receiving the Pfizer or Moderna vaccines. (Myocarditis is inflammation of the heart muscle, while pericarditis is inflammation of the heart's outer lining.)

Cases of myocarditis and/or pericarditis have been more common in adolescent boys and young men. A recent study shows that most people under age 21 who developed myocarditis thought to be linked to COVID-19 vaccination had mild symptoms that improved quickly.

Side effects can be reported to the CDC through v-safe(link opens in new window), a smartphone-based tool.

¿Debo vacunarme si estoy embarazada o si está en mis planes?

Yes — and get up to date on booster shots, too. Pregnant or recently pregnant women face an elevated risk of death or severe illness from COVID-19, including illness requiring hospitalization, intensive care, or a ventilator or other special equipment to breathe, according to the CDC. Also, risk of pregnancy complications with COVID-19 is higher.

Women who are breastfeeding or might become pregnant in the future also are urged to get vaccinated. There is no evidence that any vaccines, including those protecting against COVID-19, cause fertility problems for women or men, the CDC says.

¿Cómo puedo prepararme para mi cita de vacunación? ¿Qué debo esperar?

Si tiene la oportunidad de completar el papeleo en línea antes de la visita, ahorrará tiempo en su cita. Prevea solo una actividad ligera durante aproximadamente un día después de recibir la dosis, en caso de que tenga algún efecto a causa de la vacuna, como dolor o sensación de malestar.

Consulte con su médico o proveedor de vacunas si algún medicamento que esté tomando podría ser un problema. Los CDC aconsejan no tomar antihistamínicos ni analgésicos de venta libre como paracetamol, aspirina o ibuprofeno únicamente como medida para prevenir efectos secundarios de la vacuna.

Bring your photo ID and your insurance card (if applicable). If this is not your first shot, bring the CDC COVID-19 Vaccination Record Card(s) that you received at previous visits. Continue to practice social distancing while in line for your vaccination.

The actual shot, given in the upper arm muscle, will only take a few seconds. Afterward, you will be asked to wait 15 to 30 minutes to ensure that a risky but very rare and treatable allergic reaction, called anaphylaxis (see “side effects” question above), does not occur. Health care workers onsite are ready to treat anyone who has this reaction.

¿Puedo recibir la vacuna contra el COVID-19 al mismo tiempo que otra vacuna?

Sí. Los CDC indican que una vacuna contra el COVID-19 y otras vacunas, como la vacuna contra la gripe, se pueden administrar juntas o sin un intervalo de tiempo específico entre ellas.

Where can I get the vaccine?

Check with your local pharmacies — many are part of a federal program(link opens in new window) to increase vaccine access. You can also use the VaccineFinder tool(link opens in new window) to search for vaccine providers in your area, or check the website of your state or territorial health department(link opens in new window). The vaccine might also be available at your primary care provider's office; if not, staff there can guide you.

¿Cuáles son los efectos secundarios más comunes de la vacunación?

Muchas personas no tienen efectos secundarios de la vacuna contra el COVID-19. Muchos otros experimentan efectos menores durante algunos días, como los siguientes:

  • Dolor, enrojecimiento o inflamación en el lugar de la inyección
  • Fatiga
  • Dolor de cabeza o muscular
  • Escalofríos o fiebre
  • Náuseas

Side effects are more common after the second shot in a two-shot primary series. Call your health care team if these effects last more than a few days, are worrisome or if the redness and swelling in your arm worsens after 24 hours. For minor issues, check out this advice from the CDC(link opens in new window).

You can report side effects directly to the CDC through a smartphone-based tool called v-safe.(link opens in new window) (You will need the card you were given when you were vaccinated, which records the date and type of vaccination you received as well as the location).

¿Puedo contagiarme de COVID-19 por la vacuna?

No. Ninguna de las vacunas disponibles en los EE. UU. contiene el virus vivo. Pero debido a que las vacunas enseñan al cuerpo a reconocer y combatir el virus que causa el COVID-19, es posible que tenga efectos secundarios como fiebre, que pueden ocurrir cuando el sistema inmunitario está trabajando duro, generando su inmunidad contra el virus.

Also, it's possible you could already have COVID-19 but not be feeling symptoms before vaccination — or could catch the disease right after your shot, before your immune system can protect you. So be sure to keep following COVID-19 precautions(link opens in new window), including physical distancing and masking in communities where COVID-19 rates(link opens in new window) are high.

¿Mi examen de COVID-19 podría ser positivo debido a la vacuna?

No resultará positivo en pruebas virales, que se usan para ver si está infectado actualmente. Se recomienda realizar una prueba viral si:

  • tiene síntomas de COVID-19
  • you've had close contact(link opens in new window) with someone who has confirmed COVID-19
  • you need a test for travel, work, school, or in advance of an event or social gathering
  • un profesional de atención médica o de salud pública le indicó que se realice una prueba

You might test positive on some antibody tests, which look for signs of an immune system response to a previous infection.

¿La vacuna evitará que contagie de COVID-19 a los demás?

Research so far has suggested the vaccines can help limit the spread of COVID-19 (in addition to preventing severe illness or death in you). However, new variants continue to challenge the vaccines’ ability to prevent infection.

Wait a minute – isn’t the pandemic over? Can’t I just go back to my old lifestyle?

The term pandemic refers to the spread of a disease across several countries, affecting a large number of people. By that definition, the COVID-19 pandemic continues globally(link opens in new window).

In the U.S., many counties have "substantial" or "high" levels of COVID-19 transmission in the community (check this COVID Data Tracker(link opens in new window)). In these areas, the CDC advises fully vaccinated people, along with unvaccinated people, to wear a mask in public indoor settings (more guidance is here(link opens in new window)).

In areas with low levels of coronavirus transmission, fully vaccinated people generally don't need masks, indoors or out, with a few exceptions — for instance, if you have a weakened immune system. You also might wear a mask in public if someone in your household is immunocompromised, is not fully vaccinated or has a medical condition(link opens in new window) that increases risk for severe disease. Masks are also recommended when using public transportation and in related facilities such as airports and bus depots.

Masks are still required for fully vaccinated people in some circumstances, such as where state or local law mandates them, and in health care settings, businesses and workplaces that require masking.

Traveling is more complicated. Wearing a well-fitting mask over your nose and mouth is recommended(link opens in new window) in indoor areas of public transportation (including airplanes, trains and buses) and indoors in U.S. transportation hubs (including airports). The CDC provides detailed guidance for domestic(link opens in new window) and international(link opens in new window) travel.

Even after vaccination, you should still be alert to any COVID-19 symptoms you might have. If symptoms arise(link opens in new window), get tested, stay home, and avoid others.

¿Los niños pueden vacunarse contra el COVID-19?

Yes. The CDC recommends everyone ages 6 months and older get fully vaccinated against COVID-19. Boosters are also advised(link opens in new window) for some children – check out the CDC’s booster tool(link opens in new window) or consult with your pediatric care team.

¿Debería recibir la vacuna si tengo antecedentes de enfermedad cardíaca o accidente cerebrovascular?

Yes. People with cardiovascular risk factors, heart disease, and heart attack and stroke survivors should get vaccinated because they are at much greater risk from the virus than they are from the vaccine, according to leaders of the American Heart Association. The AHA urges people with medical conditions to discuss vaccination with their health care team.

¿La vacunación afecta otros procedimientos médicos a los que podría someterme? Escuché que las mamografías pueden ser un problema.

Si es posible, realícese la mamografía antes de la vacunación contra el COVID-19 o hable con su equipo de atención médica acerca del tiempo que debe esperar después. (Algunos expertos aconsejan esperar de cuatro a seis semanas). Como parte de la reacción inmunitaria normal que se produce después de la vacunación contra el COVID-19, es posible que tenga inflamación del ganglio linfático en la axila cerca de donde recibió la vacuna. La inflamación podría generar una falsa lectura de su mamografía.

You should also consult with your health care team about the timing of your COVID-19 vaccination if you've recently been ill or have a major medical procedure scheduled soon.

Tengo alergias. ¿Debería recibir la vacuna contra el COVID-19?

If you've had a severe or immediate allergic reaction to any ingredient(link opens in new window)— including polyethylene glycol or polysorbate — in a particular type of COVID-19 vaccine, you should not get that vaccine or not get it again. (The CDC website discusses the ingredients for the Pfizer(link opens in new window), Moderna(link opens in new window), and Johnson & Johnson(link opens in new window) vaccines at the bottom of pages discussing vaccine safety.) Ask your health care team whether you should get a different type of vaccine.

Consulte también con su equipo de atención médica si ha tenido alguna reacción alérgica inmediata a otra vacuna o tratamiento inyectable para otra enfermedad. Si tiene alergias que no están relacionadas con vacunas, incluidas alergias a alimentos, mascotas o látex, los CDC recomiendan que se vacune contra el COVID-19.

Si ya tuve COVID-19, ¿aún me debo vacunar?

Yes – vaccination will provide extra protection against another case of COVID-19. Because reinfection is less likely during a short window after having COVID-19, you may consider delaying your vaccine by three months from when symptoms started or when you tested positive. But you should take into account your personal risk and rates of COVID-19 in your community, the CDC says.

¿No estaría mejor protegido contra el COVID-19 en el futuro si me infectara naturalmente, en vez de vacunarme?

There's no way to predict how COVID-19 would affect you. Severe COVID-19 illness, and even death, have occurred in people of all ages — including healthy people. If you do get infected, vaccination helps protect you against severe illness, hospitalization and death.

Should I get a flu shot this fall?

Yes. A yearly flu shot is recommended for everyone 6 months and older, with rare exceptions. For some young children(link opens in new window), two doses are advised — check with your child’s health care team.

Influenza vaccination can help keep you and your family healthy this winter — so you might avoid a medical visit at a time when lots of people are sick and contagious. Flu vaccination is particularly important for children age 2 and under and adults 65 and older. It’s also vital for who have any of a variety of medical conditions(link opens in new window) including kidney disease, diabetes or heart failure; have a body mass index of 40 or higher; or have had a heart attack or stroke.

Discuss vaccination with your health care team if you have allergies to eggs or flu shot ingredients, have had a severe allergic reaction to a previous influenza vaccine, have a history of Guillain-Barré syndrome(link opens in new window) or are feeling ill.

The flu vaccine will not protect you from COVID-19. So if you haven’t had a COVID-19 shot yet, you can get both vaccinations at the same time, the CDC says.

¿Qué son las "variantes" del virus? ¿Pueden las vacunas contra el COVID-19 actuales proteger contra las variantes?

Cuando un virus lo infecta, se apropia de sus células para hacer numerosas copias de sí mismo que puedan infectar otras células. Se producen errores, o mutaciones, durante el proceso de copia. Una variante aparece cuando un error de copia demuestra ser beneficioso para el virus, por ejemplo, cuando lo hace más infeccioso, y se sigue realizando. Mientras más circula el virus en una población, más oportunidades hay de que se presenten mutaciones y variantes.

Variants to the virus that causes COVID-19 so far have included Alpha, Delta and the one currently of most consequence, Omicron. Subvariants, or lineages, of Omicron are currently by far the most dominant ones(link opens in new window) causing COVID-19 in the U.S.

Although the Omicron variant generally causes less severe disease than previous variants, it is also highly contagious, meaning many people may still require hospitalization when Omicron infections rise. While current vaccines provide protection against variants of the novel coronavirus, major vaccine companies are working to develop boosters specifically targeting Omicron subvariants.

¿Qué significa "inmunidad de rebaño"?

La inmunidad de rebaño se refiere al punto en el que una cantidad suficiente de personas en la sociedad tienen anticuerpos contra una enfermedad, ya sea que se hayan vacunado o infectado con anterioridad, en el que es difícil que la enfermedad se propague de persona a persona. La inmunidad de rebaño ayuda a proteger a las personas que no pueden ser vacunadas, tales como los recién nacidos o las personas que son alérgicas a la vacuna.

Experts in infectious diseases now say(link opens in new window) achieving herd immunity against the virus that causes COVID-19 is unlikely. That’s because, among other factors, the virus is continually mutating, creating new variants; it can be spread by people without symptoms (making it harder to track and control); rates of vaccination and use of other precautionary measures in the population are inadequate; and protection against new COVID-19 infection provided by previous infection or vaccination isn’t lasting.

¿Dónde puedo obtener más información sobre las vacunas contra el COVID-19?

El sitio web de los CDC incluye información exhaustiva y actualizada sobre las vacunas contra el COVID-19(el vínculo se abre en una nueva ventana).

¿Cómo puedo participar en un estudio clínico de la vacuna o el tratamiento contra el COVID-19?

El sitio web del Instituto Nacional de Alergias y Enfermedades Infecciosas contiene información sobre estudios clínicos(el vínculo se abre en una nueva ventana).

¿Qué sucede si pierdo mi tarjeta de vacunación contra el COVID-19?

If you haven't taken a photo or made a photocopy of your card as a backup, then contact your vaccination provider. If you can't reach the provider directly, check with your state health department's immunization information system(link opens in new window). (Vaccination providers are required to report COVID-19 shots to such record-keeping systems.) The CDC provides more about your vaccination card(link opens in new window).


DESCARGO DE RESPONSABILIDAD SOBRE CUIDADO DE LA SALUD: Este sitio y sus servicios no constituyen un consultorio de recomendaciones médicas, de diagnóstico ni de tratamiento. Siempre consulte a un profesional de la salud para el diagnóstico y el tratamiento, lo que incluye sus necesidades médicas específicas. Si tiene o sospecha que tiene algún problema o afección médica, comuníquese inmediatamente con un profesional de la salud calificado. Si se encuentra en Estados Unidos y tiene una emergencia médica, llame al 911 o al número de emergencia local, o solicite ayuda médica de emergencia de inmediato.