Minimum Ages & Intervals Clarified

The Oregon Immunization Program responded to a request to clarify the terms recommended and minimum in the standing orders and pharmacy protocols. The issue of spacing also came up for clarification.

Do you know the difference between the preferred (recommended) age and the (minimum) acceptable age, the preferred (recommended) spacing and the (minimum) acceptable spacing as noted in the Oregon Model Standing Orders for Immunization, the Oregon Pharmacy Protocols and the Centers for Disease Control and Prevention (CDC) Recommended Immunization Catch-up Schedules?

Look at this section from the HPV vaccine schedule:Capture.JPG- Dose and route 1


The Preferred Age and spacing are established by the manufacturer through vaccine safety trials and approved by the Advisory Committee on Immunization Practices (ACIP); a group of medical and public health experts from across the US. The ACIP determines preferred age, minimum acceptable age, preferred spacing and minimum acceptable spacing intervals based many factors, including the safety and effectiveness of a vaccine(s) and situational identifiers. The ACIP recommendations do not become policy until they are published in the Morbidity and Mortality Weekly Report (MMWR). The MMWR is the CDC’s primary vehicle for scientific publication of timely, reliable, authoritative, accurate, objective and useful public health information and recommendations.

For HPV the recommended age is 11-12 years, as this coincides with the adolescent well visit which allows plenty of time for clients to complete the 2-dose series before age 15.

The Minimum Acceptable Age and spacing are based on the expert opinion of the Advisory Committee on Immunization Practices (ACIP); which are used under exceptional circumstances whereas the preferred age and spacing are for the general population identified for a specific vaccine.

Once the MMWR is published these same recommendations are available in the Oregon Model Standing Orders for Immunization and Pharmacy Protocols.

The standing orders and pharmacy protocols are updated as soon as new recommendations are made by the ACIP. The full ACIP meets three times a year to review, discuss and vote on vaccine issues and recommendations. Subcommittees of the ACIP work throughout the year.

The CDC publishes childhood and adult guidelines and vaccine schedules annually.

Vaccinators should ensure that they are following the most current schedules from CDC.

When to use a shorter interval than Recommended Spacing1

Minimum Acceptable Spacing: Administration of a multidose vaccine series using intervals that are shorter than preferred might be necessary in certain circumstances, such as impending international travel or when a person is behind schedule on vaccinations but needs rapid protection. In these situations, an accelerated schedule can be implemented using intervals between doses that are shorter than intervals preferred for routine vaccination. The minimum acceptable spacing and ages for scheduling catch-up vaccinations are available at Vaccine doses should not be administered at intervals less than these minimum acceptable intervals or at an age that is younger than the minimum acceptable age.

There is one more exception to the minimum acceptable spacing and age and that is the 4-day rule or grace period.1

Vaccine doses administered ≤4 days before the minimum acceptable interval or age are considered valid; however, local or state mandates might supersede this 4-day guideline. Day 1 is the day before the day that marks the minimum acceptable age or minimum acceptable interval for a vaccine. Because of the unique schedule for rabies vaccine, the 4-day guideline does not apply to this vaccine. Doses of any vaccine administered ≥5 days earlier than the minimum acceptable interval or age should not be counted as valid doses and should be repeated as age appropriate. The repeat dose should be spaced after the invalid dose by the minimum acceptable interval.

If the first dose in a series is given ≥5 days before the minimum acceptable age, the dose should be repeated on or after the date when the child reaches at least the minimum acceptable age. If the vaccine is a live vaccine, ensuring that a minimum interval of 28 days has elapsed from the invalid dose is preferred.

What is Time? 2

The ACIP introduced guidelines in 2002:  If the interval is less than 4 months, it is common to covert months into days or weeks. (e.g., 1 month = 4 weeks = 28 days).

For intervals of 4 months or longer, you should consider a month a “calendar month” – the interval from one calendar date to the next a month later. (e.g., 6 months from October 1 is April 1).

Make sure you check the exact wording on the CDC’s immunization schedules.

The 4-day “grace period” should not be used when scheduling future vaccination visits, and should not be applied to the 28-day interval between live parenteral vaccines not administered at the same visit. It should be used primarily when reviewing vaccination records (for example, when evaluating a vaccination record prior to entry to daycare or school).


Sometimes outbreaks of certain vaccine preventable diseases take place. This may create exceptions to the standing orders and pharmacy protocols. Meningococcal B vaccine is one of these exceptions.

Here is a section of the vaccine schedule for an outbreak situation:Schedule for Outbreaks

And, here is the general recommendation for Meningococcal B vaccine:Schedule for Mening B JPG*The same edition of the MMWR covers both contingencies.


When you choose the age and timing of a vaccine you need to consider the optimal response.  The optimal response to a vaccine depends on multiple factors, including the type of vaccine, age of the recipient, and immune status of the recipient. Recommendations for the age at which vaccines are administered are influenced by age-specific risks for disease, age-specific risks for complications, age-specific responses to vaccination, and potential interference with the immune response by passively transferred maternal antibodies. “Vaccines are generally preferred for members of the youngest age group at risk for experiencing the disease for which vaccine efficacy and safety have been demonstrated.” This minimum age is not just for those persons aged from birth through 18 years.

Look at the new Shingrix vaccine: 

Shingrix® vaccine (RZV) by GlaxoSmithKline for immunocompetent adults at 50 years of age and older.

Schedule for Shingrix JPG

The preferred spacing interval of the first dose of Shingrix after Zostavax® (ZVL) is ≥5 years. This is the time period studied by the manufacturer and identified in the package insert. The minimum acceptable spacing between the most recent Zostavax® and the first dose of Shingrix® is 8 weeks. This has been determined by ACIP expert opinion. The MMWR states that: “Studies examined the safety and immunogenicity of RZV vaccination administered ≥5 years after ZVL; shorter intervals have not been studied… Clinical trials indicated lower efficacy of ZVL in adults aged ≥70 years; therefore, a shorter interval may be considered based on the recipient’s age when ZVL was administered. Based on expert opinion, RZV should not be given <2 months after receipt of ZVL”. The 4-day rule does not apply to the minimum spacing of Shingrix®.

What do you hope to avoid by paying close attention to age and spacing intervals?2

  • Avoid harm to the vaccinee from a side effect or vulnerability to disease;
  • Avoid the inconvenience to the parent/patient and perhaps ill will;
  • Avoid the unreimbursed cost to the provider; and
  • Avoid the loss of trust in the provider, with possible negative publicity or even legal action.

Helpful Hints:

  1. Schedule clients using the preferred age and preferred spacing to provide optimal protection.1
  2. Do not schedule your routine vaccine appointments by the minimum acceptable age, spacing or the 4-day rule. The Minimum Acceptable Age and spacing are used under exceptional circumstances, not general scheduling.
  3. Check the Oregon Model Standing Orders and the Oregon Pharmacy Protocols for the most current recommendations.


  1. Vaccine Recommendations and Guidelines of the ACIP. General Best Practice Guidelines for Immunization: Best Practice Guidance of the Advisory Committee on Immunization Practices (ACIP)

Accessed 17 April 2018

  1. Immunization Action Coalition. Available at Slide 17, 19.  Accessed 17 April 2018


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