The Pfizer-BioNTech COVID-19 vaccine requires two doses given 21 days apart to provide maximum protection against the virus. The two-dose series helps the immune system mount a robust response to the spike protein found on the surface of the SARS-CoV-2 virus. While a single dose provides some protection, health experts recommend receiving both doses for optimal efficacy against COVID-19 infection, hospitalization, and death.
What are the dosing recommendations for the Pfizer vaccine?
The Pfizer-BioNTech COVID-19 vaccine is administered as a two-dose primary series, with the doses given 21 days apart. The recommended dosing schedule is:
- First dose on day 1
- Second dose on day 21 (3 weeks after the first dose)
This 21-day interval between the first and second dose provides time for the body to build immunity after the first dose, while the second dose boosts the immune response. The two doses prime the immune system to recognize and respond to the spike protein on the coronavirus.
Why are two doses needed?
Two doses are needed to achieve maximum protection from the Pfizer COVID-19 vaccine. The first dose starts building immunity, while the second dose boosts the immune response substantially. Here’s why both doses are important:
- Priming the immune system: The first dose primes the adaptive immune system to recognize the spike protein and begin generating antibodies and activating T-cells to respond to the virus.
- Boosting immune response: The second dose given 21 days later boosts antibody levels up to 10 times higher than after the first dose. It also further stimulates T-cell activity, providing robust and likely longer-lasting immunity.
- Ensuring optimal efficacy: Two doses maximize vaccine efficacy. Clinical trials found 95% efficacy against COVID-19 infection after two doses, compared to 52% efficacy after the first dose alone.
The prime-boost approach stimulates a stronger, more sustained immune response than a single dose of the vaccine.
What protection does one dose provide?
A single dose of the Pfizer-BioNTech COVID-19 vaccine provides some protection, but this protection is minimal and short-lived compared to receiving the full two-dose series. Studies have found:
- After one dose, efficacy against COVID-19 infection is around 52% up to 12 days after the first shot.
- Efficacy against symptomatic COVID-19 disease is estimated at 57-80% after one dose.
- Partial protection begins as soon as 12 days after the first shot.
- Some studies observed waning immunity just a few weeks after the first dose.
While a single dose confers some benefit, protection is incomplete and likely to be transient without the boost from a second dose. Both doses are vital to achieve the fullest and most durable protection offered by the Pfizer vaccine.
How effective is the Pfizer vaccine after two doses?
The Pfizer-BioNTech COVID-19 vaccine provides a very high level of efficacy against COVID-19 infection, symptoms, hospitalization and death after completion of the two-dose primary series. Clinical trial data found:
- 95% efficacy against symptomatic COVID-19 infection 7 days after the second dose.
- 100% efficacy against severe COVID-19 disease.
- 95% efficacy against COVID-19 hospitalization after the second dose.
- Approximately 90% real-world effectiveness against infection and hospitalization during predominant circulation of variants like Delta and Omicron.
- Protection against COVID-19 death is estimated at around 96% after two doses.
Protection from the second dose rises rapidly to plateau at around 95% efficacy within about a week or two. Immunity then remains extremely high over the ensuing months, though some gradual waning may occur. The two doses work together to provide robust and sustained protection against developing COVID-19.
How long does protection last after getting the second dose?
Research indicates protection from the Pfizer-BioNTech COVID-19 vaccine remains high for many months after completion of the two-dose primary series. However, immunity may wane somewhat over time. Here’s what we know so far about how long protection lasts:
- Efficacy remains around 96% for at least 2 months after the second dose, with only minimal waning initially.
- Effectiveness is estimated to remain around 90% or higher for up to 6 months post-vaccination.
- Antibody levels decline gradually over the first few months after vaccination and then stabilize, providing longer-term humoral immunity.
- T-cells continue to respond robustly up to 8 months post-vaccination, indicating sustained cellular immunity.
- Breakthrough infection risk increases minimally in the first 6 months but rises more notably after 6 months.
- A booster dose restores waning immunity, with efficacy returning to about 95%.
While immunity may decline somewhat over time, protection against COVID-19 remains very high for many months after getting both doses of the Pfizer vaccine. Periodic booster shots can extend this protection further.
Should I get a booster dose?
Booster doses of the Pfizer COVID-19 vaccine are recommended to provide additional protection against waning immunity over time. The following groups are eligible for Pfizer booster shots:
- Everyone age 12 and older
- At least 5 months after completing the two-dose primary series
- Those who are moderately or severely immunocompromised may get a third primary dose earlier at least 28 days after dose 2.
A single booster dose restores immune protection to peak levels, increasing efficacy against infection and severe illness back up to about 95%. Booster shots provide extended safety amidst waning immunity and possible emergence of new variants. They help sustain the robust protection provided by the initial two-dose vaccination.
Pfizer booster efficacy data
Clinical trial results on the Pfizer booster dose show:
- 95% efficacy against symptomatic COVID-19 infection compared to those with only two doses
- 100% efficacy against severe disease after a booster dose
- Improved efficacy against the Delta and Omicron variants
The data demonstrates a booster safely restores the high level of protection achieved shortly after the initial vaccination. Boosters are an important tool for sustaining immunity over time.
Can you mix and match vaccine types for the booster dose?
The CDC allows mix and match dosing for booster shots. This means you can get a different type of COVID-19 vaccine for your booster than you originally received. Any of the approved vaccines may be used for the booster dose.
Some key points about vaccine mixing for boosters:
- mRNA vaccines (Pfizer or Moderna) are preferred regardless of which vaccine you got before.
- The single-dose J&J vaccine may be boosted with any authorized vaccine.
- It’s fine to switch vaccines for convenience or availability.
- Mixing vaccines is safe and effective for boosting immunity.
The flexibility to use different vaccines allows people to take advantage of the best available protection through boosting, even if they can’t get the same vaccine again.
Immune response when mixing vaccine types
Studies show mixing vaccines triggers comparable or sometimes stronger immune responses compared to getting the same booster as your initial shots. The immune boost is similar whether you match or mix vaccine brands.
Some observations on mixed booster dosing:
- An mRNA booster after Ad vector vaccines (J&J) produces very high antibody levels.
- Moderna after Pfizer produces higher antibody levels than repeating Pfizer.
- Pfizer after Moderna produces similar antibody boost as Moderna after Moderna.
The flexibility to mix and match for boosting takes advantage of the unique benefits of different vaccine platforms. Mixing is safe while providing strong cross-protection against COVID-19.
What are the side effects of the second dose?
Side effects are more common after the second dose of the Pfizer COVID-19 vaccine. Most side effects are mild to moderate and resolve within 1-3 days. Typical side effects after dose two include:
- Pain, swelling, redness at injection site
- Fatigue
- Headache
- Muscle pain
- Chills
- Fever
- Joint pain
- Nausea or vomiting
Side effects indicate the immune system is responding to stimulation from the vaccine. Younger adults report more side effects than older people. Acetaminophen or ibuprofen can help ease post-vaccination symptoms.
Rare but serious side effects like anaphylaxis or myocarditis are also more likely after dose two. However, these remain extremely rare. Monitoring for 15-30 minutes after vaccination is recommended to watch for signs of allergic reaction.
Why side effects increase with the second dose
Second doses lead to more frequent and pronounced side effects because the immune system is primed from the first dose. This primes the immune response to react more vigorously to another exposure to the spike protein from the vaccine. Reasons side effects increase after dose two:
- Immune cells activated by the first dose respond more robustly to the second dose.
- Memory B cells rapidly produce more antibodies, leading to stronger immune activation.
- Increased inflammation is triggered as antibody production ramps up.
- Vigorous immune stimulation transiently causes flu-like post-vaccination symptoms.
While inconvenient, short-lived side effects indicate the immune system is ramping up protection against the virus. Both doses are key for an optimal vaccine response.
Do you need two doses if you’ve already had COVID-19?
Recovered COVID-19 patients still benefit from getting fully vaccinated, including both doses of the Pfizer vaccine. Here’s guidance on vaccination after COVID-19:
- Get vaccinated even after COVID-19 infection.
- Vaccination provides additional protection for recovered patients.
- Wait until recovery and isolation period is over before getting vaccinated.
- Prior infection alone provides incomplete, waning immunity.
- Both doses are still recommended after natural infection for optimal protection.
While natural immunity does confer some benefit after COVID-19 illness, it may not provide robust or long-lasting protection on its own against reinfection. Getting fully vaccinated supplies a strong immune boost even after prior disease.
Response to vaccination after COVID-19 infection
When previously infected people are vaccinated, their immune response to vaccination differs compared to those without prior exposure. Some differences include:
- Much higher antibody levels are generated after the first vaccine dose.
- Symptoms like fever and muscle aches are more pronounced after vaccination.
- The second dose boosts immunity further but does not increase symptoms.
- Immunity is extremely high and broad after full vaccination.
While those recovered from COVID-19 may have some natural base immunity, getting fully vaccinated provides the strongest and most comprehensive protection, both for the individual and community.
Is one dose enough if supplies are limited?
In situations of extremely limited vaccine supply, delaying the second dose to vaccinate more people with a first dose may provide public health benefits. However, this strategy is not recommended routinely. Here are some considerations on single-dose vaccination:
- During early vaccine rollout, supplies may be insufficient to provide on-schedule second doses.
- First doses confer some short-term protection and may curb transmission.
- Delaying boosters extends the time to achieve optimal immunity for individuals.
- With more vaccine supply, on-time second doses should be given.
- Routine single-dosing is not advised due to suboptimal, transient immunity.
While Mass single-dose deployment may benefit communities when faced with extremely limited initial supplies, two doses remain essential for robust protection at the individual level once sufficient vaccine is available.
Potential benefits of initially delaying second doses
During early vaccine rollout when supplies are very limited, delaying second doses has some potential advantages:
- More people can get partial protection faster with first doses only.
- May slow disease spread and outbreaks on a community level.
- Provides short-term priming of the immune system.
- Extends vaccine supply when it’s urgently needed.
This strategy needs to be balanced against leaving individuals inadequately protected short-term and risk of waning single-dose immunity. It’s not recommended to intentionally delay boosters once supply is sufficient.
Can you get by with just one dose after having COVID-19?
It’s still recommended for recovered COVID-19 patients to receive both doses of the Pfizer vaccine, even though they may have some natural immunity. Here’s why:
- Reinfections are still possible after an initial case of COVID-19.
- Immunity from natural infection could wane faster than vaccine immunity.
- One dose after COVID-19 provides good but not great protection.
- The second dose increases protection to very high levels.
- Variants may escape natural immunity, while vaccination provides broader protection.
Those who’ve had COVID-19 still benefit from the robust, long-lasting immunity provided by both doses of the vaccine. The second dose is important for ensuring the strongest and longest-lasting protection possible.
Comparing one vs. two doses after COVID-19 infection
Studies indicate while one dose does boost immunity among those previously infected, two doses are still needed for optimal protection:
- After 1 dose, efficacy was around 60-80%, lower than in COVID-naive people.
- After 2 doses, efficacy reached over 90%, on par with fully vaccinated COVID-naive people.
- Prior infection plus 1 dose offers moderate protection; 2 doses gives very high protection.
- Two doses further boost antibody levels and cellular immunity compared to one dose alone.
While people previously infected may gain reasonable protection from one dose, two doses are still warranted in this group for the highest level of immunity against COVID-19 reinfection.
Conclusion
In conclusion, the Pfizer-BioNTech COVID-19 vaccine is designed and clinically proven to provide maximum protection against COVID-19 when given as a two-dose regimen spaced 21 days apart. The second dose boosts the immune response substantially, conferring around 95% efficacy against the virus. While a single dose confers partial protection, two doses are essential to generate robust, sustained immunity on both an individual and community level. All eligible people are urged to complete the full vaccination course with both doses of the Pfizer vaccine plus a booster when recommended for optimal protection against COVID-19.
