Several ghost puns (popular at Halloween) involve a “booster” (“boo-ster”).
   
“Q: What kind of seat does a baby ghost sit in? A: A booster seat” was printed in the Ottawa (ON) Citizen on March 7, 1992. “What does a baby ghost need at a restaurant? A BOO-ster seat” was posted on Twitter by @FoundingEditor on October 31, 2009.
 
“Why did the ghost go to the doctor’s office? ... He had to get his booster shot” was printed in Boys’ Life on January 1997. “Why did the ghost go to the doctor? To get his boo-ster shot” was posted on the newsgroup alt.humor on October 29, 1999.
   
“What kind of COVID vaccine did the ghost get? A boo-ster!” was posted on Reddit—Dadjokes on October 27, 2021.
   
 
Wikipedia: COVID-19 pandemic
The COVID-19 pandemic, also known as the coronavirus pandemic, is an ongoing pandemic of coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), first identified in December 2019 in Wuhan, China. The outbreak was declared a Public Health Emergency of International Concern in January 2020 and a pandemic in March 2020.
     
Newspapers.com
7 March 1992, Ottawa (ON) Citizen, “Monstrous Jokes,” pg. K3, col. 1:
Q: What kind of seat does a baby ghost sit in?
A: A booster seat.
(Submitted by Julie Summers, 5, of Ottawa)
 
Google Books
January 1997, Boys’ Life, “Think & Grin,” pg. 53, col. 3:   
Terry: Why did the ghost go to the doctor’s office?
Jerry: Why?
Terry: He had to get his booster shot. — Chris McFarlin, Aurora, Ohio.
     
Newspapers.com
18 November 1998, Los Angeles (CA) Times, “The Kids’ Reading Room: Pint-Sized Punch Line,” pg. D7, col. 5:
Why was the little ghost crying in the doctor’s office? He wanted to give her a boo-ster shot. (Katrina Martinez, 9, Norwalk, Chavez Elementary)
 
Google Groups: alt.humor   
JOKE MAIL
MCant23790
Oct 29, 1999, 3:00:00 AM
(...)
Why did the ghost go to the doctor?
To get his boo-ster shot?
     
Google Groups: alt.humor
Halloween Jokes
dcoble
Oct 20, 2000, 3:00:00 AM
(...)
Q. Why did the ghost go to the doctor?
A. To get his boo-ster shot?
 
Google Groups: aus.jokes
Why did the ghost go to the doctor?
Cerberus Jokes Mailing List
Oct 30, 2000, 4:04:42 PM
Why did the ghost go to the doctor?
To get his boo-ster shot?
       
Google Books
Spooky Creature Riddles
By Janet Nuzum Myers
New York, NY: Sterling Publishing Company, Inc.
2005
Pg. 7:
Where does a baby ghost sit?
In a booster seat.
 
Twitter
Shannon Holmes, Realtor at RealtySouth
@LuvHouses
Q. Why did the ghost go to the doctor?
A. To get his boo-ster shot?
12:23 PM · Oct 27, 2009·Twitter Web Client
   
Twitter
Editor-in-Chief ラケル　Ｄ　ベリー
@FoundingEditor
What does a baby ghost need at a restaurant?
A BOO-ster seat
3:12 PM · Oct 31, 2009·Twitter Web Client
   
Twitter
Daily Groaner
@DailyGroaner
Q: What kind of seat do baby ghosts sit in? A: A Boo-ster seat. #jokes #humor #funny
12:33 PM · Feb 3, 2011·Twitter Web Client
 
Twitter
Bob Kostic
@causticbob
Q: Why did the ghost go the the doctor? A: To get his Boo-ster shot
5:35 PM · Oct 18, 2011·TweetDeck
   
Twitter
DoctorHumerus
@DoctorHumerus
Q. Why did the baby ghost go to the #pediatrician?
A. To get his boo-ster shot! #jokes #HappyHalloween
12:54 PM · Oct 31, 2011·Twitter Web Client
   
Twitter
Phoebs
@sweetphoeb
@1DCyberpunk
What does a baby ghost need at a restaurant? A boo-ster seat! #TRICKORTWEET1D☻
4:20 PM · Oct 31, 2011·Twitter Web Client
   
Twitter
Breezy
@bre_vandeusen
Why did the ghost go to the doctor? He needed a BOO-ster shot!
7:34 PM · Oct 2, 2014·Twitter for Windows Phone
   
Twitter
Olive⁷
@OliviaALeaf
what do baby ghosts sit in? boo-ster seats
7:22 PM · Oct 20, 2014·Twitter for iPhone
   
Twitter
Paul
@jammasta_pmoney
What does a baby ghost need at a restaurant?
A boo-ster seat
6:19 PM · Oct 31, 2016·Twitter for iPhone
       
Twitter 
Natasha Ghoulseph
@TashJoeZA
The American sent me a terrible joke and insists it’s funny: Why wasn’t the ghost afraid of Covid?
Because he got his boo-ster shot.
5:04 AM · Oct 9, 2021·Twitter for Android
 
Twitter
Shannon Shackleford
@ShannonShackl14
Why did the ghost go to the doctor?
To get a BOO-ster shot
12:30 AM · Oct 17, 2021·Twitter for Android
 
Twitter
Pat Morrison
@PatMorr5188
Why did the ghost go to Walgreens?
To get his BOO-ster.
7:15 PM · Oct 23, 2021·Twitter for Android
 
Reddit—Dadjokes
Posted by u/Wellhellotherebeans October 27, 2021
What kind of COVID vaccine did the ghost get?
A boo-ster!