Update: On March 29, 2022, the Food and Drug Administration (FDA) authorized a second booster dose (4th dose) for those ages 50 and older (and for immunocompromised individuals). We have added the 50+ scenario to our analysis.

With Congress having reached a funding stalemate regarding additional COVID-19 support, the White House has said that the federal government “does not have adequate resources to purchase enough booster vaccine doses for all Americans, if additional doses are needed.”  We sought to assess this by estimating how far the current U.S. supply of COVID-19 vaccines could stretch under different scenarios. To do so, we looked at the number of COVID-19 vaccine doses the federal government has already purchased, the number of people already vaccinated and boosted. We then estimated the number of additional doses that would be required to fill any gaps needed to reach vaccination targets, including to fully vaccinate those who still haven’t completed a primary series, under different assumptions about further booster coverage with 3rd and 4th doses. We assumed that primary vaccination will soon be authorized for all ages, and examined the future supply of mRNA vaccine doses only (i.e., Johnson and Johnson doses were not included). We assessed the following four scenarios:

  1. Providing a 4th dose to those ages 50 and older
  2. Providing a 4th dose to those ages 65 and older
  3. Providing a 4th dose to those ages 18 and older
  4. Providing a 4th dose to all ages

For each scenario, we looked at supply needed to purchase enough doses to reach two vaccine coverage targets: 70% and 100%. Data are as of March 20, 2022 (see methods for sources and additional details).


  • We find that the federal government does not have enough vaccine doses remaining in its current supply to fully cover the U.S. population with a 4th dose under any of these scenarios.
  • The deficit between current supply and doses required is most acute (484 million doses) if a 4th dose is authorized for all ages and enough doses are purchased to cover 100% of the population. However, a deficit still exists even if 4th doses are needed only for those ages 65 and older (162.5 million doses), as well as for the larger age group of people 50 and older (a deficit of 225 million doses), as authorized by the FDA on March 29..
  • When a 70% population coverage target is used instead, there is still a supply deficit (118.3 million doses) if 4th doses are purchased for those of all ages, and a modest deficit to reach 70% of those ages 18 and older with a 4th dose (34 million). There are enough doses to cover 70% of those ages 65 and older with a 4th dose as well as 70% of those ages 50 and older.
  • Based on the lowest available prices paid per dose by the U.S. government for domestic vaccine supply ($15.00/dose for Moderna and $19.50 for Pfizer), we estimate that additional funding required ranges from $509.5 million (if all future doses were Moderna and enough were purchased to reach 70% coverage of those 18 and older with a 4th dose) to $9.4 billion (if all future doses were Pfizer and enough were purchased to reach 100% of all ages with a 4th dose).

With the FDA authorizing a 4th dose for those ages 50 and older, our analysis finds that the U.S. government currently has purchased enough vaccine doses for 70% of this age group to receive a 4th dose, and for completing vaccination for others. While the government does not have enough doses to cover all those ages 50 and older, it is also unlikely that 4th dose take-up will approach 100% of the population anytime soon. If 4th dose authorization is further expanded, potential supply constraints will grow.

Table 1: Number of Additional COVID-19 Vaccine Doses Needed by Scenario and Coverage Target
Scenario 70% Coverage 100% Coverage
4th dose to 50+         240,419,688           528,522,614
4th dose to 65+         196,460,245           465,723,410
4th dose to 18+         337,162,096           666,726,055
4th dose to all ages         421,517,894           787,234,338
Table 2: Number of Surplus/Deficit COVID-19 Vaccine Doses by Scenario and Coverage Target
Scenario 70% Coverage 100% Coverage
4th dose to 50+          62,775,437         (225,327,489)
4th dose to 65+         106,734,880         (162,528,285)
4th dose to 18+          (33,966,971)         (363,530,930)
4th dose to all ages        (118,322,769)         (484,039,213)

We calculated the number of people who would need to be vaccinated in the U.S. to reach 70% and 100% vaccine coverage by age group. We compared these levels to the number who had already completed a primary COVID-19 vaccine series (if they received two doses of the two-dose mRNA series or one dose of a single-dose vaccine), and the number who had received a 3rd dose (booster) by age group. We then looked at the additional doses needed for four scenarios: (1) providing a 4th dose to those ages 50 and older 2) providing a 4th dose to those ages 65 and older; (3) providing a 4th dose to those ages 18 and older; and (4) providing a 4th dose to all ages. For each scenario, we assessed the number of doses needed to complete the primary vaccine series and, for scenarios 1-3, the number of doses needed to provide a third dose to all those ages 12 and older, as is currently recommended by CDC, before adding a 4th dose. For scenario 4, we assumed a 3rd dose would be recommended for all ages. For each of the 4 scenarios, we examined both 70% and 100% coverage targets, resulting in 8 distinct supply requirement outcomes. We then compared the number of additional doses needed in each supply outcome to the number of doses remaining in the current U.S. vaccine supply, and determined the amount of supply surplus or deficit. We assumed that any doses already delivered (whether administered yet or not) were no longer available for purposes of the remaining supply. We also did not include remaining J&J vaccines in the available future supply and we removed Moderna doses already provided for international use from the available supply. Finally, we estimated the potential cost of purchasing additional doses if more were needed. We used the estimated price paid per dose for Moderna and Pfizer vaccines using the lowest available price per dose estimate paid to date among supply contracts. Data on the size of the U.S. population by age group were obtained from the U.S. Census. Data on the number of U.S. vaccine doses purchased and estimated price per dose were obtained from official U.S. government, pharmaceutical company releases, and the UNICEF COVID-19 Vaccine Market Dashboard. Data on the number of vaccine doses delivered and number of people vaccinated were obtained from the CDC COVID Data Tracker and are current as of March 20, 2022.

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