Deployed app cannot connect to Pubmed API and returns Error 429

I have a Shiny app that works fine locally, and that I was able to deploy on shinyappsio.
However, when running it, it returns a connection error (429 which upon searching for it means "Too many requests").
The app uses the RISmed package which enables to connect to and query the Pubmed database.

Is there any way to solve for this? It seems that the function from the package cannot connect from shinyappsio to the NCBI API.

Below are the error statements from the log (I replace the email with ***).

Thank you

Warning in file(con, "r") :  cannot open URL '***': HTTP status was '429 Unknown Error'

Warning: Error in file: cannot open the connection to '***'

Perhaps it's hitting an API limit when deployed? i.e. maybe it's running the requests faster?

Just a guess - I've had to slow down a lot of my API requests (although I've not used NCBI's).