WITH medians AS (
    SELECT
        country,
        date_id,
        vaccines,
        daily_vaccinations,
        PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY daily_vaccinations) OVER (PARTITION BY country) AS country_median
    FROM
        mytable
),
  
filled_data AS (
    SELECT
        country,
        date_id,
        vaccines,
        COALESCE(daily_vaccinations, country_median) AS daily_vaccinations
    FROM
        medians
)

SELECT country,
       date_id,
       vaccines,
       ISNULL(daily_vaccinations, 0) AS filled_vaccinations
FROM filled_data;