こちらで行ったのと同じ計算を Go で行いました。
Python3: 接種状況ダッシュボードのデータを使ってワクチン接種済者をカウントする

count_vaccination.go

// ----------------------------------------------------------------
//
//	count_vaccination.go
//
//					Jan/26/2022
//
// ----------------------------------------------------------------
package main 

import (
	"fmt"
	"os"
	"log"
	"encoding/csv"
//	"reflect"
	"strconv"
	"time"
) 

// ----------------------------------------------------------------
func get_day_6months_proc () string {
	now := time.Now ()
	fmt.Printf ("%s\n" ,now)
	today := strconv.Itoa (now.Year()) + "-" +
		fmt.Sprintf ("%02d",now.Month()) + "-" +
		strconv.Itoa (now.Day())

	t3 := now.AddDate (0,0,-184)
	day_6months := strconv.Itoa (t3.Year()) + "-" +
		fmt.Sprintf ("%02d",t3.Month()) + "-" +
		strconv.Itoa (t3.Day())

	fmt.Println(today)
	fmt.Println(day_6months)

	return	day_6months
}

// ----------------------------------------------------------------
func count_proc(record [][]string,day_6months string){
	llx := len(record)

	second := 0
	second_6months := 0
	third := 0


	for it := 1; it < llx; it++{
		ss,_ := strconv.Atoi(record[it][2])
		second  += ss
		tt,_ := strconv.Atoi(record[it][3])
		third  += tt
		if (record[it][0] < day_6months){
			second_6months += ss
			}
	}

	fmt.Printf ("second = %d\n",second)
	fmt.Printf ("second_6months = %d\n",second_6months)
	fmt.Printf ("third = %d\n",third)
}


// ----------------------------------------------------------------
func main (){
	fmt.Fprintf (os.Stderr,"*** 開始 ***\n")

	file_csv := os.Args[1]
	fmt.Printf (file_csv + "\n")

	ff, err := os.Open(file_csv)
	if err != nil {
		log.Fatal(err)
	}

	rr := csv.NewReader(ff)

	record, err := rr.ReadAll()
	if err != nil {
		log.Fatal(err)
	}

	fmt.Printf("%#v\n", len(record))
//	fmt.Println(reflect.TypeOf(record))

	day_6months := get_day_6months_proc()

	count_proc(record,day_6months)

	fmt.Fprintf (os.Stderr,"*** 終了 ***\n")
}

// ----------------------------------------------------------------

実行結果

$ go run count_vaccination.go summary_by_date.csv
*** 開始 ***
summary_by_date.csv
290
2022-01-26 19:50:18.261192179 +0900 JST m=+0.000425766
2022-01-26
2021-07-26
second = 93846301
second_6months = 29646353
third = 2892327
*** 終了 ***