Removing county boundaries with usmap function

Question

I am using the usmap package in R to create a heat map of the United States. My data are at the county/fips level. I have successfully rendered them into a map but would like to remove the county boarders and leave in the state boarders. Does anybody know how to do this?

Data: Download

Code:

  mapdata <- read.csv("File location here",header = T)
  yr <- 2008
  yrstr <- toString(yr)

  # Subset to relevant year and variables
  yrdata <- subset(mapdata, year == yr)
  yrdata1 <- yrdata[,c("fips_code","eligibles")]
  colnames(yrdata1)[1] <- "fips"

  plot_usmap("states", data = yrdata1, value = "eligibles", color = "white", size = 0) + 
    scale_fill_continuous(low = "blue", high = "red", name = paste0("Medicare Eligible Population (",yrstr,")"), label = scales::comma) +
    theme(legend.position = "right")