WITH medians AS (
    SELECT
        country,
        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,
        COALESCE(daily_vaccinations, country_median) AS filled_daily_vaccinations
    FROM
        medians
)
SELECT *
FROM filled_data;