Adjusted earnings per diluted share, excluding foreign currency impact, increased 24.2% for the quarter and 24.5% for the year.
At the same time, sales improved year-over-year for the first time during the pandemic in the second quarter in both the United States and Japan.
Looking at our operations in Japan, in the second quarter, Aflac Japan generated solid financial growth results as reflected in its profit margin of 26.5%.
Aflac Japan also reported strong premium persistency of 94.7%.
Sales improved year-over-year, generating an increase of 38.4% for the quarter and 15.7% for the first six months.
While sales in the first half of 2021 are at approximately 65% of 2019 levels, we continue to navigate evolving pandemic conditions in Japan.
Turning to Aflac U.S., we saw a strong profit margin of 24.4 -- 25.4% and very strong premium persistency of 80.1%.
As a result of softer sales a year earlier and more face-to-face opportunities, sales increased 64.1% for the quarter and are at a 73% of the first half of the 2019 levels.
Medical product sales for the first half of the year are up roughly 48% over the same period in 2020 and have approached pre-pandemic levels down only 4% from the 2019 period.
In the second quarter, we have registered close to 600 agencies with SUDACHI and issued about 230 policies.
In the second quarter, we have processed over 14,000 online applications as compared to nearly 8,000 in the first quarter.
On Japan Post, proposal activity has increased month-to-month as sales training and promotion permeates the 20,000 branches that sell our insurance.
Through the month of June, Aflac Japan has conducted over 35,000 training sessions with Japan Post sales agents nationwide, along with providing contact information on nearly 700,000 existing cancer policyholders to inform on the latest coverage advantages.
Individual agent recruiting remains under pressure, and we are running at 70% of weekly producers we enjoyed pre-pandemic.
Our Network Dental product is approved in 43 states and Vision in 42 states, with more states coming online later in the year.
We are completing national training programs and have about 50% of trained agents who have quoted on our new dental and vision product offerings.
It's early, however, we continue to see our volume building each month, and over 50% of our dental and vision cases include voluntary benefit sales.
With respect to our e-commerce initiative, Aflac Direct, we offer critical illness, accident and cancer and are now approved in 45 states, including California.
Our digital platform overall is experiencing a 16% conversion rate on qualified leads and generally consistent with many digital D2C insurance platforms.
Through six months in 2021, these three platforms, new platforms, have combined for 5% of sales as they are in the early building and development stages.
We continue to forecast a strong second half based on increased activity and expect these three growth initiatives will contribute upwards of 15% to sales in the second half of 2021.
Aflac has made a $2 billion multiyear general account commitment to launch a new debt platform focused on investing in the senior secured debt of sustainable infrastructure projects.
Aflac will hold a 24.9% minority interest in a newly created entity Denham Sustainable Infrastructure.
We are also making a $100 million commitment to Denham Equity Fund, focused on sustainable infrastructure investments.
For the second quarter, adjusted earnings per share increased 24.2% to $1.59.
This strong performance for the quarter was largely driven by lower claims utilization due to the pandemic, especially in the U.S. In addition, variable investment income ran $112 million above our long-term return expectations.
Adjusted book value per share, including foreign currency translation gains and losses, grew 20.5%.
And the adjusted ROE, excluding foreign currency impact, was a strong 17%, which is a significant spread to our cost of capital.
Starting with our Japan segment, total earned premium for the quarter declined 3.8%, reflecting the impact of first sector policies reaching paid-up status, while earned premium for our third sector products was down 2.3% due to recent low sales volumes.
Japan's total benefit ratio came in at 66.9% for the quarter, down 290 basis points year-over-year.
And the third sector benefit ratio was 56.5%, down 305 basis points year-over-year.
Persistency was down 10 basis points, yet remained strong at 94.7%.
Our adjusted expense ratio in Japan was 20.8%, up 80 basis points year-over-year.
Adjusted net investment income increased 27.4% in yen terms, primarily driven by favorable returns on our growing alternatives portfolio and lower hedge costs, partially offset by lower reinvestment yield on our fixed rate portfolio.
The pre-tax margin for Japan in the quarter was 26.5%, up 450 basis points year-over-year, which was a very favorable result for the quarter.
This quarter's strong financial results leads us to expect the full year benefit ratio for Japan to be at the lower end of the 3-year guidance range of 68.5% to 71% given at Fab.
And the pre-tax margin to be at the higher end of the 20.5% to 22.5% range.
Turning to U.S. results, net earned premium was down 3.4% due to weaker sales results.
Persistency improved 180 basis points to 80.1%.
63 basis points of the elevated persistency in both the second quarter of this year and the prior year can be explained by emergency orders.
80 basis points of improved persistency in the quarter is attributed to lower sales, as first year lapse rates are roughly twice total in-force lapse rates.
Another 30 basis points of improved persistency is due to conservation efforts, and the remainder largely comes from improved experience.
Our total benefit ratio came in lower than expected at 43.5% or 80 basis points lower than Q2 2020, which, itself, was heavily impacted by the initial pandemic.
This quarter, IBNR releases amounted to 5.6 percentage points impact on the benefit ratio, which leads to an underlying benefit ratio, excluding IBNR releases, of 49.1%.
For the full year, we now expect our benefit ratio to be in the range of 45% to 48% versus our original guidance of 48% to 51%.
Our expense ratio in the U.S. was 36.9%, up 160 basis points year-over-year but with a lot of moving parts.
Higher advertising spend increased the expense ratio by 60 basis points.
Our continued buildout of growth initiatives, group life and disability, network dental and vision and direct to consumer contributed to a 170 basis point increase to the ratio.
In the quarter, we also incurred $5.5 million of integration and transition expenses not included in adjusted earnings associated with recent acquisitions.
Adjusted net investment income in the U.S. was up 9.9%, mainly driven by favorable variable investment income in the quarter.
Profitability in the U.S. segment was very strong with a pre-tax margin of 25.4%, with a low benefit ratio as the core driver.
Initial expectations were for us to be toward the low end of 16% to 19%.
In our Corporate segment, we recorded a pre-tax loss of $76 million, as adjusted net investment income was $45 million lower than last year due to low interest rates at the short end of the yield curve and amortization of certain tax credit investments, which amounted to $30 million this quarter held at the corporate level.
Our capital position remains strong, and we ended the quarter with an SMR above 900% in Japan and an RBC of approximately 600% in Aflac Columbus.
Unencumbered holding company liquidity stood at $4.4 billion, which was $2 billion above our minimum balance, excluding the $400 million proceeds from the sustainability bond that we issued in March that reinforced our ESG initiatives and believe that sustainable investments are also good long-term investments.
Leverage, which includes our sustainability bond, remains at a comfortable 22.8%, in the middle of our leverage corridor of 20% to 25%.
In the quarter, we repurchased $500 million of our own stock and paid dividends of $223 million, offering good relative IRR on these capital deployments.

Summary:
At the same time, sales improved year-over-year for the first time during the pandemic in the second quarter in both the United States and Japan.
For the second quarter, adjusted earnings per share increased 24.2% to $1.59.
This quarter, IBNR releases amounted to 5.6 percentage points impact on the benefit ratio, which leads to an underlying benefit ratio, excluding IBNR releases, of 49.1%.