Portal shields (often just called shields) are mods which make a portal’s resonators more resistant to enemy attacks.

Adding shielding to a portal contributes to its mitigation, which is the percentage of damage from enemy attacks which is blocked, making the portal’s resonators more resistant. A portal’s total mitigation (from shields and links together) is capped at 95; adding more will not increase it beyond that level.

Type Shielding Stickiness
Common +30 0
Rare +40 150,000
Very Rare +60 450,000
Aegis +70 800,000

The table below shows all combinations of installed shields that do not exceed the 95 mitigation ceiling (not taking links into account):

Mod 1 Mod 2 Mod 3 Shielding
0
Common 30
Common Common 60
Common Common Common 90
Rare 40
Rare Common 70
Rare Rare 80
Very Rare 60
Very Rare Common 90
Aegis 70

 

Aegis Shield

The Aegis shield (originally branded as the AXA shield) is a variant of the very rare shield. It was introduced on 15 December 2014 through a sponsorship deal with the international investment banking firm AXA, and that variant is adorned with their logo. They are the strongest and stickiest type of shield available. They can be hacked from any portal, but the AXA shield was more likely to be dropped by portals at AXA locations.[source]

On 11 December 2017, Niantic revealed that the AXA shield was to be discontinued, and a new, unbranded shield called the Aegis shield would be released. With the release of version 1.129.2, all AXA shields were converted to Aegis shields, and the AXA shield no longer exists.