What do we know (and not know) about delaying the second vaccine dose in kids?

Families/Kids Vaccines

Here’s the short answer:

➡️ We DO know that the standard three week interval between first and second Pfizer-BioNTech doses is safe and effective in kids ages 5-11. It has been well tested and gives your child very good protection as soon as possible.

➡️ We DON’T know the safety or efficacy of delaying the second dose in kids. Real-world studies in adults suggest that stronger and longer protection are possible, but we don’t have data in kids. Some experts theorize that a longer gap between doses could reduce the already small risk of myocarditis but not all experts are on the same page, and we don’t have published data on this question.

⚖️ These uncertain benefits may or may not be outweighed by the risks that come with waiting. Let’s take a closer look:


💪 POSSIBLY stronger and longer protection. Several studies in adults have found that a longer interval between doses provided a stronger immune response (eg. far higher levels of “neutralizing antibodies”) and better protection against infection and hospitalization. It’s reasonable to assume the same is true in kids, but we don’t have hard data yet. Some experts believe that a higher peak level of protection will translate to longer protection, but this is unproven. Since the 3 week interval already offers excellent protection from infection (90%) that lasts for months, any benefit would be subtle.

💗 POSSIBLY a lower risk of vaccine-induced myocarditis. Canada’s NACI said that this risk “may” be lower based on unpublished “Emerging Canadian Surveillance data” (notably, not in this age group). Nobody truly knows how a different dose spacing will impact the risk of myocarditis in kids at this point, and not every immunologist is on the same page with what to expect. In our nerdy opinion, this potential benefit is very shaky.

📉How low are we talking? Myocarditis rates in kids ages 5-11 who get vaccinated are likely similar, or even lower, than the rates in 12-15 year olds (1 per 25,000 males and 1 per 250,000 females per CDC’s estimates). It would be wonderful to drop from very very low to very very very low rates, but it’s not a sure thing.


If you are contemplating delaying your child’s second vaccine dose, be sure to consider the following risks:

🦠 Getting a preventable case of COVID-19 while you await full protection. When kids get infected, it’s very disruptive to families, even when kids are free of symptoms. Kids that get COVID-19 are also at risk of having long COVID and missing school (no, please no more virtual school!!!!). While the chances of serious health issues due to COVID-19 are small for most children without comorbidities, the risk posed by vaccination is far smaller.

😬 Spreading a preventable case of COVID-19. When a child gets COVID-19, the parents and siblings often get it too, as some of the Nerdy Girls know all too well from personal experience. Family members may unknowingly spread it to their contacts, and so on. At a population level, particularly as new variants emerge, vaccinating children sooner could decrease the total cases, hospitalizations, and deaths.

The chances of a preventable case depend on several factors, including some unknowns:

❓ Your child’s level of exposure to COVID-19. This is shaped both by your local case rates, vaccination rates, and your family’s jobs, activities, and travel plans. Notably, our exposure risk can change rapidly in a short period of time as the COVID-19 landscape changes.

❓ Whether or not your child already had COVID-19. Natural infection likely offers some level of protection, but getting vaccinated is recommended for all kids as the best way to ensure consistent, lasting protection.

❓How much protection a single dose offers. We don’t know how much protection a single dose offers, especially for kids. We do know, however, that two doses always provide far better protection than one. For example, a large UK study of vaccine effectiveness against the delta variant found 30% lower rates of symptomatic disease after one dose but 88% after two doses.


The last piece of the puzzle is a bit of context on why we are even talking about veering from the validated three week regimen.

➡️ Short intervals were used in the mRNA clinical trials because the world was (and still is!) on fire 🔥with COVID 🔥 and we needed to develop a vaccine and reach full vaccination as quickly as possible. No one wanted to wait around for 4 or 5 months to find out if the vaccines worked. In non-pandemic times, most multi dose vaccines space doses by months, not weeks.

➡️ Longer intervals were initially used in some countries out of necessity – due to lack of supply (e.g. in Canada and the UK). Normally, public health agencies follow the regimen tested in clinical trials because we understand its safety and efficacy.

➡️ Canada’s National Advisory Committee on Immunization (NACI) recommends spacing doses in kids (and adults) eight weeks or more apart. They cited the potential benefits described above based on data in Canada and Europe, and noted that longer intervals are “consistent with principles of vaccinology”. While Health Canada approved the three week dose regimen tested in trials, provinces are expected to heed NACI’s advice.

➡️ NACI noted the importance of context in their guidelines: “When choosing to use a longer dose interval, local transmission of the SARS-CoV-2 virus; the degree of individual risk of exposure to the virus; and the need of a second dose for earlier protection should be considered.”

➡️ Canada’s context is currently far less dire than the current US situation. Canada has high vaccination rates (89% of those 12 and over, 78% of total population) and modest community transmission rates (e.g. 92 cases per 100K over the last two weeks, compared to 384 per 100k in the US).


💥Welcome to public health! We make the best decisions we can, for our context, with the information we have.

We hope that this discussion helps you frame your decision for your family and your context. Sticking with the standard three week regimen is safe and effective, and offers a huge mental benefit: relief! It’s worth a lot to know that your child is well protected, and is far less likely to bring SARS CoV-2 home. There are some potential benefits to delaying the second dose, but they are uncertain, and may or may not outweigh the costs of waiting. Whatever you do, be sure to get the full protection offered by that second dose.

FWIW, at least two of the nerdy girls are following the approved three week timeline for their kids, and at least one of them is going to delay the second dose. The one who plans to delay had COVID last year (along with her children). Chana (yours truly) is in Canada, where eight weeks or more between doses is the status quo, but subject to change if our context changes dramatically.

DP links:

A Nerdy Girl and her family get COVID-19!

Want to know more about infection vs. vaccine induced immunity to SARS-CoV-2?


Canada’s NACI recommendations for COVID-19 vaccines in kids 5-11

Canada’s NACI guidelines for adults and youths

mRNA COVID-19 Vaccine-Associated Myocarditis, ACIP Presentation by Matthew Oster, MD, MPH

Immunogenicity of standard and extended dosing intervals of BNT162b2 mRNA vaccine

Higher serological responses and increased vaccine effectiveness demonstrate the value of extended vaccine schedules in combatting COVID-19 in England

Two-dose SARS-CoV-2 vaccine effectiveness with mixed schedules and extended dosing intervals: test-negative design studies from British Columbia and Quebec, Canada

Strong humoral immune responses against SARS-CoV-2 Spike after BNT162b2 mRNA vaccination with a 16-week interval between doses

Effectiveness of Covid-19 Vaccines against the B.1.617.2 (Delta) Variant

Association of Age and Pediatric Household Transmission of SARS-CoV-2 Infection

What COVID vaccines for young kids could mean for the pandemic

Vaccine rates in Canada

COVID-19 case rates per country per 100K

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