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The results come hot on the heels of similar results from Pfizer, and add to growing confidence that vaccines can help end the pandemic.
Both companies used a highly innovative and experimental approach to designing their vaccines.
Moderna says it is a "great day" and they plan to apply for approval to use the vaccine in the next few weeks.
However, this is still early data and key questions remain unanswered.
We still do not know how long immunity will last as volunteers will have to be followed for much longer before that can be answered.
There is also no data on how well it works in older age groups, who are most at risk of dying from Covid.
And it is not known whether the vaccine just stops people becoming severely ill, or if it stops them spreading the virus too.
All these questions will affect how a coronavirus vaccine is used.
Good news, but with the usual caveat .....
How does this compare to the Pfizer vaccine?
Both vaccines use the same approach of injecting part of the virus's genetic code in order to provoke an immune response.
The preliminary data we have seen so far is very similar - around 90% protection for the Pfizer vaccine and around 95% for Moderna's.
However, both trials are still taking place and the final numbers could change.
Moderna's vaccine appears to be easier to store as it remains stable at minus 20C for up to six months and can be kept in a standard fridge for up to a month.
Pfizer's vaccine needs ultra-cold storage at around minus 75C, but it can be kept in the fridge for five days.
Matt Hancock has suggested ministers could make it a requirement if initial take-up is lower than expected.
A change in the law will probably be introduced to coerce those "anti-vaxxers" who are mugs enough to believe misinformation posted to social platforms .....
I read somewhere today that after 'a poll' something like 54% of people said they wouldn't have the vaccine yet as they are afraid of side-effects while it is so new.
That's me Mups .... I'm going to hold off. I don't especially want the vaccine and wonder if enough other people have it that it won't matter much if I don't.